On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃
The temperature dependences of the unit cell parameters a(T) and c(T) of Ag₃AsS₃ were measured by the X-ray dilatometry method with high precision within the temperature range 100 to 300 K in the dark mode and under laser irradiation (λ = 532 nm). It was found that the parameter c increases al...
Gespeichert in:
| Datum: | 2013 |
|---|---|
| Hauptverfasser: | , , , |
| Format: | Artikel |
| Sprache: | English |
| Veröffentlicht: |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2013
|
| Schriftenreihe: | Semiconductor Physics Quantum Electronics & Optoelectronics |
| Online Zugang: | http://dspace.nbuv.gov.ua/handle/123456789/117735 |
| Tags: |
Tag hinzufügen
Keine Tags, Fügen Sie den ersten Tag hinzu!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Zitieren: | On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ / N. Borovoy, Yu. Gololobov, G. Isaienko, A. Salnik // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2013. — Т. 16, № 3. — С. 293-296. — Бібліогр.: 12 назв. — англ. |
Institution
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-117735 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1177352017-05-27T03:03:09Z On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ Borovoy, N. Gololobov, Yu. Isaienko, G. Salnik, A. The temperature dependences of the unit cell parameters a(T) and c(T) of Ag₃AsS₃ were measured by the X-ray dilatometry method with high precision within the temperature range 100 to 300 K in the dark mode and under laser irradiation (λ = 532 nm). It was found that the parameter c increases almost linear with decreasing the temperature from 300 down to 100 K for samples in the dark. At the same time, for samples that were exposed during cooling to laser irradiation, the increase of the parameter с by the value ∆c ≈ (0.002…0.003) Å is observed at temperatures Tp = 145…147 K. This leap is typical for systems in which a phase transition of the first order occurs. Furthermore, there were investigated temperature dependences of the integral relative intensity I(T) of main structural maxima of Ag₃AsS₃ both in the dark mode and under laser irradiation. It was established the character of the dependence I(T) for these reflexes was changed significantly by laser irradiation. 2013 Article On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ / N. Borovoy, Yu. Gololobov, G. Isaienko, A. Salnik // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2013. — Т. 16, № 3. — С. 293-296. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 61.50.Ks, 77.80.Bh http://dspace.nbuv.gov.ua/handle/123456789/117735 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
The temperature dependences of the unit cell parameters a(T) and c(T) of
Ag₃AsS₃ were measured by the X-ray dilatometry method with high precision within the
temperature range 100 to 300 K in the dark mode and under laser irradiation (λ =
532 nm). It was found that the parameter c increases almost linear with decreasing the
temperature from 300 down to 100 K for samples in the dark. At the same time, for
samples that were exposed during cooling to laser irradiation, the increase of the
parameter с by the value ∆c ≈ (0.002…0.003) Å is observed at temperatures Tp =
145…147 K. This leap is typical for systems in which a phase transition of the first order
occurs. Furthermore, there were investigated temperature dependences of the integral
relative intensity I(T) of main structural maxima of Ag₃AsS₃ both in the dark mode and
under laser irradiation. It was established the character of the dependence I(T) for these
reflexes was changed significantly by laser irradiation. |
| format |
Article |
| author |
Borovoy, N. Gololobov, Yu. Isaienko, G. Salnik, A. |
| spellingShingle |
Borovoy, N. Gololobov, Yu. Isaienko, G. Salnik, A. On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Borovoy, N. Gololobov, Yu. Isaienko, G. Salnik, A. |
| author_sort |
Borovoy, N. |
| title |
On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ |
| title_short |
On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ |
| title_full |
On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ |
| title_fullStr |
On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ |
| title_full_unstemmed |
On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ |
| title_sort |
on features of crystal structure of semiconductor-ferroelectric ag₃ass₃ |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2013 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/117735 |
| citation_txt |
On features of crystal structure of semiconductor-ferroelectric Ag₃AsS₃ / N. Borovoy, Yu. Gololobov, G. Isaienko, A. Salnik // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2013. — Т. 16, № 3. — С. 293-296. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT borovoyn onfeaturesofcrystalstructureofsemiconductorferroelectricag3ass3 AT gololobovyu onfeaturesofcrystalstructureofsemiconductorferroelectricag3ass3 AT isaienkog onfeaturesofcrystalstructureofsemiconductorferroelectricag3ass3 AT salnika onfeaturesofcrystalstructureofsemiconductorferroelectricag3ass3 |
| first_indexed |
2025-07-08T12:42:52Z |
| last_indexed |
2025-07-08T12:42:52Z |
| _version_ |
1837082687658000384 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 3. P. 293-296.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
293
PACS 61.50.Ks, 77.80.Bh
On features of crystal structure
of semiconductor-ferroelectric Ag3AsS3
N. Borovoy1, Yu. Gololobov2, G. Isaienko2, A. Salnik1*
1Taras Shevchenko Kyiv National University, Faculty of Physics,
4, prosp. Glushkova, Kyiv, Ukraine; *e-mail: alina.salnik@gmail.com
2National Transport University, Faculty of Transport and Information Technologies,
1, Suvorova str., 01011 Kyiv, Ukraine
Abstract. The temperature dependences of the unit cell parameters a(T) and c(T) of
Ag3AsS3 were measured by the X-ray dilatometry method with high precision within the
temperature range 100 to 300 K in the dark mode and under laser irradiation (λ =
532 nm). It was found that the parameter c increases almost linear with decreasing the
temperature from 300 down to 100 K for samples in the dark. At the same time, for
samples that were exposed during cooling to laser irradiation, the increase of the
parameter с by the value ∆c ≈ (0.002…0.003) Å is observed at temperatures Tp =
145…147 K. This leap is typical for systems in which a phase transition of the first order
occurs. Furthermore, there were investigated temperature dependences of the integral
relative intensity I(T) of main structural maxima of Ag3AsS3 both in the dark mode and
under laser irradiation. It was established the character of the dependence I(T) for these
reflexes was changed significantly by laser irradiation.
Keywords: proustite, unit cell parameters, phase transition, photoinduced effects,
integral intensity.
Manuscript received 09.07.13; revised version received 06.09.13; accepted for
publication 19.09.13; published online 30.09.13.
1. Introduction
The crystals of proustite Ag3AsS3 are semiconductors-
ferroelectrics with high ionic conductivity, which are
characterized by large optical anisotropy, pleochroism
and transparency in infrared region. Because of these
properties, proustite belongs to promising materials of
nonlinear optics. That is why its physical properties are
actively investigated within the temperature range 4.2 to
600 K [1, 2]. The crystals of proustite belong to the
noncentro-symmetrical class of trigonal symmetry at
room temperature (space group 6
3vC ) [2-4]. Temperature
decrease of proustite is accompanied by structural phase
transformations (PT), according to [3] at temperatures of
56 K (symmetry changing C3vCs) and 24 K (CsC1).
Full-scale study of the heat capacity and the
character of temperature transformation of NQR
spectrum allowed authors [5] to firstly conclude about
the existence of II order PT at Ті = (60.50.5) K and
I order PT at ТN = (49.50.5) K in crystals of proustite
and about the existence of an incommensurate phase
between these PT. After that, formation of modulated
structures were directly confirmed by X-ray diffraction
and neutronographic methods at Ті = 60 K [6, 7]. Further
investigations showed that the modulated structure is
also incommensurate in the interval Т = 28…50 K, but
with rather a small period and orientation changes [8].
So, there is some sequence of PTs at temperatures Т <
100 K in crystals Ag3AsS3: symmetric phase 60 K
incommensurate phase 50 K new incommensurate
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 3. P. 293-296.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
294
phase 28 K ferroelectric phase without structural
modulation. There are different viewpoints about PT at
temperatures Т > 100 K in proustite in the literature.
The important feature of proustite is its crystal
structure sensitivity to optical irradiation. Particularly,
authors [3] first suggested existence of photoinduced
changes at the temperature Т ≈ 150 K in proustite
crystals Ag3AsS3 based on revealed splitting of the E-
mode in the Raman spectra. The results of Raman
scattering and Brillouin scattering investigations enabled
to conclude about existence of the photoinduced PT in
the temperature range Т = 200…210 K and propose
another scheme of structural transformations: C3v Cs
(PT of the II order, Т0 = 210 K), Cs Cs (a cell
doubling, PT of the II order, Т1 = 56 K), Cs C1 (PT of
the I order, Т2 = 24 K) [4, 9, 10]. But, the anomalies that
are typical for PT, were observed in the temperature
dependences of the velocity of longitudinal ultrasonic
waves in irradiated proustite samples at the temperature
T ≈ 150 K [11].
So, there is no single viewpoint about character and
mechanisms of temperature and optical irradiation effect
on physical properties of proustite crystals in the
literature. Furthermore, the precision measurements of
the unit cell parameter a and c of proutite crystals at
temperature T < 300 K were not represented in the
literature at all. Therefore, in this work the temperature
dependences of the unit cell parameters a(T) and c(T)
were investigated using the X-ray dilatometry method,
also determined were integral relative intensities of some
main structural maxima of proustite crystals within the
temperature range 100<T<300 K both in the dark mode
and under laser irradiation.
2. Experimental
The investigated samples were grown by the Bridgman-
Stockbarger method. The X-ray diffraction patterns of
Ag3AsS3 were obtained using DRON-4-07
diffractometer (Fe Kα1,2 radiation). To study the
dependences a(T) and c(T), single crystals of proustite
were used, in which planes (00l) and (h00) were derived
on the physical surface. To obtain the dependence c(T),
the reflection (006) was registered, and in case a(T) –
reflection (10.00).
Reflections were scanned in steps (2θ) = 0.02˚, the
accumulation of impulses at a point was 2 s. The
functions of Voigt, which fitted the compounds Kα1 and
Kα2, were used during separation of Kα1,2–doublet. The
accuracy in determining the lattice parameters a and c
was not worse than 0.002 Å. The study of the
temperature dependences of the relative integral
intensity I(T) of structural maxima were performed on
polycrystalline samples that were obtained by grinding
single crystals Ag3AsS3. The diffraction patterns of
polycrystals were obtained in some angular intervals of
the width ∆(2θ) = (3…5)º, which corresponded to the
structural maxima (122), (232), (312) and (202) in steps
(2θ) = 0.03º, the accumulation time at the point was
10…30 s. Samples were placed in a URNT-180 standard
low-temperature chamber. The sample temperature was
maintained with the accuracy 0.5 K using a VRT-2
thermoregulator and measured by copper-constantan
thermocouple. Optical irradiation of samples during
cooling and heating was carried out using the MGL 500
milliwatt laser (λ = 532 nm), irradiation power incident
on the sample was 40 mW.
3. Results and discussions
Fig. 1 shows temperature dependences of the unit cell
parameter a(T), which were obtained during samples
cooling: (a) – dark mode; (b) – under laser irradiation.
As can be seen, dependences a(T) are close to
linear, and the value of parameter a is almost the same in
the dark mode and under laser irradiation. At the same
time, there was observed a weak change in the slope of
curve a(T) at the temperatures T = 170…200 K on both
figures, so the slope angle is smaller at T > 200 K than
T < 170 K. It should be noted that dependences obtained
during heating almost do not differ from the above.
100 120 140 160 180 200 220 240 260 280 300
10.37
10.38
10.39
10.40
10.41
T,K
a,A
0
(a)
100 120 140 160 180 200 220 240 260 280 300
10.37
10.38
10.39
10.40
10.41
a,A
0
(b)
T,K
Fig. 1. Temperature dependences a(T) for Ag3AsS3: a) dark
mode; b) under laser irradiation.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 3. P. 293-296.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
295
Temperature dependence of the unit cell parameter
c(T) of proustite crystals both in the dark mode and
under laser irradiation of the sample (cooling mode) is
shown in Fig. 2a and 2b, respectively. First of all, there
are two temperature ranges with significantly different
slope angles of c(T): interval ∆T1 = 220…300 K, where
the parameter c in general slightly varies with
temperature, and interval ∆T2 = 100…170 K, where the
slope angle is significant. The slope angle of the
dependence c(T) is monotonically changed within the
temperature range 170 to 220 K. Thus, the coefficient of
thermal expansion Tcc of Ag3AsS3 crystals
along the axis C was significantly different in two
temperature ranges ∆T1 and ∆T2:
K/Å105.3 5
1
Tc ; K/Å104.1 4
2
Tc .
It should be noted that in the paper [1], the
parameters a and c were determined with poor
experimental accuracy, so the changing of angle slope
c(T) was not observed within the temperature range 170
to 220 K. It is important that the character of the
dependence c(T) is practically the same for both samples
in the dark mode and under laser irradiation.
100 120 140 160 180 200 220 240 260 280 300 320
8.704
8.708
8.712
8.716
8.720
T,K
c,A
0
(a)
100 120 140 160 180 200 220 240 260 280 300
8.704
8.708
8.712
8.716
8.720
A
0
c,
T,K
(b)
Fig. 2. Temperature dependences c(T) for Ag3AsS3: a) dark
mode; b) under laser irradiation.
100 120 140 160 180 200 220 240 260 280 300
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
T, K
I, arb. un.
1
2
Fig. 3. Temperature dependences I(T) for reflexion (312):
1) dark mode; 2) under laser irradiation.
However, there observed was an important
difference between them for temperatures Т < 170 K.
Namely, parameter c monotonically increases with
temperature decreasing for samples within the
temperature range 100 to 170 K in the dark. At the same
time, there is an abrupt increase of the parameter ∆c ≈
0.002…0.003 Å at temperatures 144 to 146 K for
samples that were laser irradiated during the cooling
process. This leap of unit cell parameters values is
typical for systems in which phase transitions of the first
order occur.
We have previously established that dependences
I(T) for reflections (122), (232), (312) and (202) are
fundamentally different for polycrystalline samples of
proustite in the dark mode in the course of cooling in
two temperature regions: integral intensities for all four
maxima decrease from room temperature down to
140…160 K (an abnormal dependences), and with
further cooling down to 100 K integral intensities grow
[12]. In this work, the temperature dependences I(T)
were measured for mentioned maxima both in the dark
mode and under laser irradiation. It was revealed that the
character of the dependence I(T) significantly change:
integral intensities for all maxima increase within the
temperature range 300 to 100 K during the cooling
process. In Fig. 3 this effect is illustrated for the
reflection (312).
The nature of revealed photoinduced effects – like
leap in the dependences c(T) at the temperature T =
144…146 K and differences in the dependence I(T), may
be related to the process of photostimulated migration of
Ag+ ions on crystallographic positions in the unit cell
(the depth of the potential well for Ag+ in the unit cell is
less than ∆U = 0.7…1.0 eV [1, 2]). The analysis of the
structure amplitude in the dark mode of proustite showed
that there are transitions of Ag+ ions at temperatures Т
150 K from main crystallographic to vacant positions, in
general right-screwed spirals (AgS). In particular, it
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 3. P. 293-296.
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
296
causes abnormal behavior of dependences I(T) for
maxima (122), (232), (312) and (202). However, the
simultaneous action of laser irradiation and temperature
leads to disordering cationic sublattice of proutite at Т
150 K, i.e. accelerates the output of silver ions with
equal probability from both right- and left-screwed
spirals (AgS). In this case, Ag+ ions can move to vacant
positions within the unit cell, or go in a structure
heterogeneity area and on the surface of crystallites.
This disordering the cationic sublattice should be
accompanied by decrease in the integral intensity of
these reflections with temperature increasing, as
observed in this work. The temperature match of the leap
in the dependence c(T) and inflection of the dependence
I(T) (Т 145…150 K) is an additional evidence that the
character of Ag+ ions migration qualitatively changes
between right- and left-screwed spirals (AgS) exactly in
this temperature range.
It should also be noted that contradictory data
known in the literature about the existence of
photoinduced changes in physical parameters of
proustite within the temperature interval 200 to 210 K
[1, 2] may be caused by the “transitional” state of
proustite crystals within the temperature range 170 to
220 K. Indeed, the coefficient of thermal expansion of
proutite αc varies more than three times within the
mentioned temperature range, which is typical for the
blurred phase transition. Therefore, optical irradiation of
proustite crystals in this state, depending on parameters
of a defect subsystem, may cause appearance of various
anomalies in temperature dependences of electrical,
thermal and optical properties.
4. Conclusions
Photoiduced leaped increasing of the unit cell parameter
c was experimentally observed at the temperature T =
(144…146 K for the first time, which confirmed the
existence of photoinduced phase transition of the first
order in proustite crystals. It was experimentally
revealed the existence of two temperature ranges –
220…300 K and 100…170 K, in which the value of the
thermal expansion coefficient for proustite crystals
changed thrice. The change in temperature dependences
of integral intensities was revealed under laser
irradiation for several structural maxima of proustite
crystals, which are caused by the action of optical
irradiation on processes of thermal migration of silver
ions in the proustite unit cell.
References
1. K.A. Schonau and S.A.T. Derfern, High-
temperature phase transitions, dielectric relaxation,
and ionic mobility of proustite, Ag3AsS3, and
pyrargyrite, Ag3SbS3 // J. Appl. Phys. 92(12),
p. 7415-7424 (2002).
2. A. Gagor, A. Pawowski and A. Pietraszko, Silver
transfer in proustite Ag3AsS3 at high temperatures:
Conductivity and single-crystal X-ray studies // J.
Solid State Chem. 182, p. 451-456 (2009).
3. G.A. Smolenskiy, I.G. Siniy, E.G. Kuzminov and
A.A. Godovikov, Optical phonons and soft mode in
proustite at phase transition // Phys. Solid State
21(8), p. 2338-2341 (1979).
4. G.A. Smolenskiy, I.G. Siniy, S.D. Prohorova, E.G.
Kuzminov and A.V. Godovikov, New phase
transition in proustite // Crystalographia 27(1),
p. 140-145 (1982), in Russian.
5. A.B. Bondar, V.S. Vihrin, S.M. Riabchenko and
V.E. Iachmenev, Incommensurate phase near phase
transition of the second order in proustite // Phys.
Solid State 25(9), p. 2602-2609 (1983).
6. S.S. Hasanov, V.Sh. Shlehtman and I.M. Shmytko,
Creation of modulated structure in proustite // Phys.
Solid State 26(3), p. 935-938 (1984).
7. R.J. Nelmest, C.J. Howard, T.W. Ryan, W.I.F.
David, A.J. Schultz and P.C.W. Leung, A neutron
and diffraction study of the phase transitions in
proustite (Ag3AsS3) between 35 K and room
temperature // J. Phys. C: Solid State Phys. 26(3),
p. 861-865 (1984).
8. I.M. Shmytko, V.Sh. Shlehtman, B.Sh.
Bagautdinov and N.S. Afonikova, Dinamic
hysteresis effects near existence of modulated
structure in proustite // Phys. Solid State 32(8),
p. 2441-2447 (1990).
9. L.A. Kot, S.D. Prohorova, Iu.M. Sandler, I.G. Siniy
and I.N. Flerov, Photostimulated phase transition in
proustite // Phys. Solid State 28(5), p. 1535-1537
(1983).
10. Ia. Shauren and K.N. Teylor, Studing og photo-
stimulated phase transition in proustite Ag3AsS3 //
Phys. Solid State 28(9), p. 2604-2607 (1986).
11. Iu.P. Gololobov, On photostimulated phase
transition in Ag3AsS3 crystal // Phys. Solid State
41(4), p. 702-704 (1999).
7. N.A. Borovoy, Iu.P. Gololobov and I.N. Salivonov,
Termorearrangement of cation sublattice in
proustite // Low Temp. Phys. 25, p. 546-549 (1999).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2013. V. 16, N 3. P. 293-296.
PACS 61.50.Ks, 77.80.Bh
On features of crystal structure
of semiconductor-ferroelectric Ag3AsS3
N. Borovoy1, Yu. Gololobov2, G. Isaienko2, A. Salnik1*
1Taras Shevchenko Kyiv National University, Faculty of Physics,
4, prosp. Glushkova, Kyiv, Ukraine; *e-mail: alina.salnik@gmail.com
2National Transport University, Faculty of Transport and Information Technologies,
1, Suvorova str., 01011 Kyiv, Ukraine
Abstract. The temperature dependences of the unit cell parameters a(T) and c(T) of Ag3AsS3 were measured by the X-ray dilatometry method with high precision within the temperature range 100 to 300 K in the dark mode and under laser irradiation (λ = 532 nm). It was found that the parameter c increases almost linear with decreasing the temperature from 300 down to 100 K for samples in the dark. At the same time, for samples that were exposed during cooling to laser irradiation, the increase of the parameter с by the value ∆c ≈ (0.002…0.003) Å is observed at temperatures Tp = 145…147 K. This leap is typical for systems in which a phase transition of the first order occurs. Furthermore, there were investigated temperature dependences of the integral relative intensity I(T) of main structural maxima of Ag3AsS3 both in the dark mode and under laser irradiation. It was established the character of the dependence I(T) for these reflexes was changed significantly by laser irradiation.
Keywords: proustite, unit cell parameters, phase transition, photoinduced effects, integral intensity.
Manuscript received 09.07.13; revised version received 06.09.13; accepted for publication 19.09.13; published online 30.09.13.
1. Introduction
The crystals of proustite Ag3AsS3 are semiconductors-ferroelectrics with high ionic conductivity, which are characterized by large optical anisotropy, pleochroism and transparency in infrared region. Because of these properties, proustite belongs to promising materials of nonlinear optics. That is why its physical properties are actively investigated within the temperature range 4.2 to 600 K [1, 2]. The crystals of proustite belong to the noncentro-symmetrical class of trigonal symmetry at room temperature (space group
6
3
v
C
) [2-4]. Temperature decrease of proustite is accompanied by structural phase transformations (PT), according to [3] at temperatures of 56 K (symmetry changing C3v(Cs) and 24 K (Cs(C1).
Full-scale study of the heat capacity and the character of temperature transformation of NQR spectrum allowed authors [5] to firstly conclude about the existence of II order PT at Ті = (60.5(0.5) K and I order PT at ТN = (49.5(0.5) K in crystals of proustite and about the existence of an incommensurate phase between these PT. After that, formation of modulated structures were directly confirmed by X-ray diffraction and neutronographic methods at Ті = 60 K [6, 7]. Further investigations showed that the modulated structure is also incommensurate in the interval Т = 28…50 K, but with rather a small period and orientation changes [8]. So, there is some sequence of PTs at temperatures Т < 100 K in crystals Ag3AsS3: symmetric phase ( 60 K ( incommensurate phase ( 50 K ( new incommensurate phase ( 28 K ( ferroelectric phase without structural modulation. There are different viewpoints about PT at temperatures Т ( 100 K in proustite in the literature.
The important feature of proustite is its crystal structure sensitivity to optical irradiation. Particularly, authors [3] first suggested existence of photoinduced changes at the temperature Т ≈ 150 K in proustite crystals Ag3AsS3 based on revealed splitting of the E-mode in the Raman spectra. The results of Raman scattering and Brillouin scattering investigations enabled to conclude about existence of the photoinduced PT in the temperature range Т = 200…210 K and propose another scheme of structural transformations: C3v ( Cs (PT of the II order, Т0 = 210 K), Cs ( Cs (a cell doubling, PT of the II order, Т1 = 56 K), Cs ( C1 (PT of the I order, Т2 = 24 K) [4, 9, 10]. But, the anomalies that are typical for PT, were observed in the temperature dependences of the velocity of longitudinal ultrasonic waves in irradiated proustite samples at the temperature T ≈ 150 K [11].
So, there is no single viewpoint about character and mechanisms of temperature and optical irradiation effect on physical properties of proustite crystals in the literature. Furthermore, the precision measurements of the unit cell parameter a and c of proutite crystals at temperature T < 300 K were not represented in the literature at all. Therefore, in this work the temperature dependences of the unit cell parameters a(T) and c(T) were investigated using the X-ray dilatometry method, also determined were integral relative intensities of some main structural maxima of proustite crystals within the temperature range 100<T<300 K both in the dark mode and under laser irradiation.
2. Experimental
The investigated samples were grown by the Bridgman-Stockbarger method. The X-ray diffraction patterns of Ag3AsS3 were obtained using DRON-4-07 diffractometer (Fe Kα1,2 radiation). To study the dependences a(T) and c(T), single crystals of proustite were used, in which planes (00l) and (h00) were derived on the physical surface. To obtain the dependence c(T), the reflection (006) was registered, and in case a(T) – reflection (10.00).
Reflections were scanned in steps ((2θ) = 0.02˚, the accumulation of impulses at a point was 2 s. The functions of Voigt, which fitted the compounds Kα1 and Kα2, were used during separation of Kα1,2–doublet. The accuracy in determining the lattice parameters a and c was not worse than 0.002 Å. The study of the temperature dependences of the relative integral intensity I(T) of structural maxima were performed on polycrystalline samples that were obtained by grinding single crystals Ag3AsS3. The diffraction patterns of polycrystals were obtained in some angular intervals of the width ∆(2θ) = (3…5)º, which corresponded to the structural maxima (122), (232), (312) and (202) in steps ((2θ) = 0.03º, the accumulation time at the point was 10…30 s. Samples were placed in a URNT-180 standard low-temperature chamber. The sample temperature was maintained with the accuracy 0.5 K using a VRT-2 thermoregulator and measured by copper-constantan thermocouple. Optical irradiation of samples during cooling and heating was carried out using the MGL 500 milliwatt laser (λ = 532 nm), irradiation power incident on the sample was 40 mW.
3. Results and discussions
Fig. 1 shows temperature dependences of the unit cell parameter a(T), which were obtained during samples cooling: (a) – dark mode; (b) – under laser irradiation.
As can be seen, dependences a(T) are close to linear, and the value of parameter a is almost the same in the dark mode and under laser irradiation. At the same time, there was observed a weak change in the slope of curve a(T) at the temperatures T = 170…200 K on both figures, so the slope angle is smaller at T > 200 K than T < 170 K. It should be noted that dependences obtained during heating almost do not differ from the above.
1
0
0
1
2
0
1
4
0
1
6
0
1
8
0
2
0
0
2
2
0
2
4
0
2
6
0
2
8
0
3
0
0
1
0
.
3
7
1
0
.
3
8
1
0
.
3
9
1
0
.
4
0
1
0
.
4
1
T
,
K
a
,
A
0
(
a
)
100120140160180200220240260280300
10.37
10.38
10.39
10.40
10.41
T,K
a,
A
0
(a)
1
0
0
1
2
0
1
4
0
1
6
0
1
8
0
2
0
0
2
2
0
2
4
0
2
6
0
2
8
0
3
0
0
1
0
.
3
7
1
0
.
3
8
1
0
.
3
9
1
0
.
4
0
1
0
.
4
1
a
,
A
0
(
b
)
T
,
K
100120140160180200220240260280300
10.3710.3810.3910.4010.41a,A0(b)T,K
Fig. 1. Temperature dependences a(T) for Ag3AsS3: a) dark mode; b) under laser irradiation.
Temperature dependence of the unit cell parameter c(T) of proustite crystals both in the dark mode and under laser irradiation of the sample (cooling mode) is shown in Fig. 2a and 2b, respectively. First of all, there are two temperature ranges with significantly different slope angles of c(T): interval ∆T1 = 220…300 K, where the parameter c in general slightly varies with temperature, and interval ∆T2 = 100…170 K, where the slope angle is significant. The slope angle of the dependence c(T) is monotonically changed within the temperature range 170 to 220 K. Thus, the coefficient of thermal expansion
T
c
c
¶
¶
=
a
of Ag3AsS3 crystals along the axis C was significantly different in two temperature ranges ∆T1 and ∆T2:
(
)
K
/
Å
10
5
.
3
5
1
-
×
=
D
a
T
c
;
(
)
K
/
Å
10
4
.
1
4
2
-
×
=
D
a
T
c
.
It should be noted that in the paper [1], the parameters a and c were determined with poor experimental accuracy, so the changing of angle slope c(T) was not observed within the temperature range 170 to 220 K. It is important that the character of the dependence c(T) is practically the same for both samples in the dark mode and under laser irradiation.
1
0
0
1
2
0
1
4
0
1
6
0
1
8
0
2
0
0
2
2
0
2
4
0
2
6
0
2
8
0
3
0
0
3
2
0
8
.
7
0
4
8
.
7
0
8
8
.
7
1
2
8
.
7
1
6
8
.
7
2
0
T
,
K
c
,
A
0
(
a
)
100120140160180200220240260280300320
8.704
8.708
8.712
8.716
8.720
T,K
c,
A
0
(a)
1
0
0
1
2
0
1
4
0
1
6
0
1
8
0
2
0
0
2
2
0
2
4
0
2
6
0
2
8
0
3
0
0
8
.
7
0
4
8
.
7
0
8
8
.
7
1
2
8
.
7
1
6
8
.
7
2
0
A
0
c
,
T
,
K
(
b
)
100120140160180200220240260280300
8.704
8.708
8.712
8.7168.720A0c,T,K
(b)
Fig. 2. Temperature dependences c(T) for Ag3AsS3: a) dark mode; b) under laser irradiation.
1
0
0
1
2
0
1
4
0
1
6
0
1
8
0
2
0
0
2
2
0
2
4
0
2
6
0
2
8
0
3
0
0
0
.
2
0
.
3
0
.
4
0
.
5
0
.
6
0
.
7
0
.
8
0
.
9
1
.
0
1
.
1
1
.
2
T
,
K
I
,
a
r
b
.
u
n
.
1
2
100120140160180200220240260280300
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
T, K
I,arb. un.
1
2
Fig. 3. Temperature dependences I(T) for reflexion (312): 1) dark mode; 2) under laser irradiation.
However, there observed was an important difference between them for temperatures Т < 170 K. Namely, parameter c monotonically increases with temperature decreasing for samples within the temperature range 100 to 170 K in the dark. At the same time, there is an abrupt increase of the parameter ∆c ≈ 0.002…0.003 Å at temperatures 144 to 146 K for samples that were laser irradiated during the cooling process. This leap of unit cell parameters values is typical for systems in which phase transitions of the first order occur.
We have previously established that dependences I(T) for reflections (122), (232), (312) and (202) are fundamentally different for polycrystalline samples of proustite in the dark mode in the course of cooling in two temperature regions: integral intensities for all four maxima decrease from room temperature down to 140…160 K (an abnormal dependences), and with further cooling down to 100 K integral intensities grow [12]. In this work, the temperature dependences I(T) were measured for mentioned maxima both in the dark mode and under laser irradiation. It was revealed that the character of the dependence I(T) significantly change: integral intensities for all maxima increase within the temperature range 300 to 100 K during the cooling process. In Fig. 3 this effect is illustrated for the reflection (312).
The nature of revealed photoinduced effects – like leap in the dependences c(T) at the temperature T = 144…146 K and differences in the dependence I(T), may be related to the process of photostimulated migration of Ag+ ions on crystallographic positions in the unit cell (the depth of the potential well for Ag+ in the unit cell is less than ∆U = 0.7…1.0 eV [1, 2]). The analysis of the structure amplitude in the dark mode of proustite showed that there are transitions of Ag+ ions at temperatures Т ( 150 K from main crystallographic to vacant positions, in general right-screwed spirals (AgS)(. In particular, it causes abnormal behavior of dependences I(T) for maxima (122), (232), (312) and (202). However, the simultaneous action of laser irradiation and temperature leads to disordering cationic sublattice of proutite at Т ( 150 K, i.e. accelerates the output of silver ions with equal probability from both right- and left-screwed spirals (AgS)(. In this case, Ag+ ions can move to vacant positions within the unit cell, or go in a structure heterogeneity area and on the surface of crystallites.
This disordering the cationic sublattice should be accompanied by decrease in the integral intensity of these reflections with temperature increasing, as observed in this work. The temperature match of the leap in the dependence c(T) and inflection of the dependence I(T) (Т ( 145…150 K) is an additional evidence that the character of Ag+ ions migration qualitatively changes between right- and left-screwed spirals (AgS)( exactly in this temperature range.
It should also be noted that contradictory data known in the literature about the existence of photoinduced changes in physical parameters of proustite within the temperature interval 200 to 210 K [1, 2] may be caused by the “transitional” state of proustite crystals within the temperature range 170 to 220 K. Indeed, the coefficient of thermal expansion of proutite αc varies more than three times within the mentioned temperature range, which is typical for the blurred phase transition. Therefore, optical irradiation of proustite crystals in this state, depending on parameters of a defect subsystem, may cause appearance of various anomalies in temperature dependences of electrical, thermal and optical properties.
4. Conclusions
Photoiduced leaped increasing of the unit cell parameter c was experimentally observed at the temperature T = (144…146 K for the first time, which confirmed the existence of photoinduced phase transition of the first order in proustite crystals. It was experimentally revealed the existence of two temperature ranges – 220…300 K and 100…170 K, in which the value of the thermal expansion coefficient for proustite crystals changed thrice. The change in temperature dependences of integral intensities was revealed under laser irradiation for several structural maxima of proustite crystals, which are caused by the action of optical irradiation on processes of thermal migration of silver ions in the proustite unit cell.
References
1. K.A. Schonau and S.A.T. Derfern, High-temperature phase transitions, dielectric relaxation, and ionic mobility of proustite, Ag3AsS3, and pyrargyrite, Ag3SbS3 // J. Appl. Phys. 92(12), p. 7415-7424 (2002).
2. A. Gagor, A. Pawowski and A. Pietraszko, Silver transfer in proustite Ag3AsS3 at high temperatures: Conductivity and single-crystal X-ray studies // J. Solid State Chem. 182, p. 451-456 (2009).
3. G.A. Smolenskiy, I.G. Siniy, E.G. Kuzminov and A.A. Godovikov, Optical phonons and soft mode in proustite at phase transition // Phys. Solid State 21(8), p. 2338-2341 (1979).
4. G.A. Smolenskiy, I.G. Siniy, S.D. Prohorova, E.G. Kuzminov and A.V. Godovikov, New phase transition in proustite // Crystalographia 27(1), p. 140-145 (1982), in Russian.
5. A.B. Bondar, V.S. Vihrin, S.M. Riabchenko and V.E. Iachmenev, Incommensurate phase near phase transition of the second order in proustite // Phys. Solid State 25(9), p. 2602-2609 (1983).
6. S.S. Hasanov, V.Sh. Shlehtman and I.M. Shmytko, Creation of modulated structure in proustite // Phys. Solid State 26(3), p. 935-938 (1984).
7. R.J. Nelmest, C.J. Howard, T.W. Ryan, W.I.F. David, A.J. Schultz and P.C.W. Leung, A neutron and diffraction study of the phase transitions in proustite (Ag3AsS3) between 35 K and room temperature // J. Phys. C: Solid State Phys. 26(3), p. 861-865 (1984).
8. I.M. Shmytko, V.Sh. Shlehtman, B.Sh. Bagautdinov and N.S. Afonikova, Dinamic hysteresis effects near existence of modulated structure in proustite // Phys. Solid State 32(8), p. 2441-2447 (1990).
9. L.A. Kot, S.D. Prohorova, Iu.M. Sandler, I.G. Siniy and I.N. Flerov, Photostimulated phase transition in proustite // Phys. Solid State 28(5), p. 1535-1537 (1983).
10. Ia. Shauren and K.N. Teylor, Studing og photostimulated phase transition in proustite Ag3AsS3 // Phys. Solid State 28(9), p. 2604-2607 (1986).
11. Iu.P. Gololobov, On photostimulated phase transition in Ag3AsS3 crystal // Phys. Solid State 41(4), p. 702-704 (1999).
7. N.A. Borovoy, Iu.P. Gololobov and I.N. Salivonov, Termorearrangement of cation sublattice in proustite // Low Temp. Phys. 25, p. 546-549 (1999).
© 2013, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
293
_1441462857.unknown
_1442752390.unknown
_1442752393.unknown
_1441460061.unknown
|