- Email: [email protected]
On the nature of the low-temperature phase of NaD3(SeO3)2
J. Phys. Chem. Solids
Pergamon Press 1969. Vol. 30, pp. 57-62.
Printed in Great Britain.
ON THE NATURE OF THE LOW-TEMPERATURE PHASE OF NaD,(SeO,), L. A. SHUVALOV, N. R. IVANOV, L. F. KIRPICHNIKOVA Institute of Crystallography
and N. M. SCHAGINA
of the Academy of Sciences of the USSR, Moscow, USSR
(Received 14 December 1967; in revisedform 3 May 1968) Abstract- Dielectric and optical properties of NaDJSeO& single crystals were studied. From the results obtained the low-temperature phase of NaD,(SeO,), is considered to be ferroelectric. It is shown that the phase transition is attended with 2/m + m symmetry change associated with disappearance on strong deuteration of the interstitial triclinic ferroelectric phase which was found to exist for NaH,(SeO,),.
by us coincides with the system of coordinates which we had used previously for NaH,(SeO& crystals [7]. This orientation differs from that developed by Blinc ei al.[4] so that our x-, y- and z-axes, being parallel to the main axes of the optical indica&ix, coincide with the directions -X cos 36”, Y, -2 cos 36” reported in [4]. Measurements were carried out using plates 0.5-l mm thick and 1 cm2 in area. Low-frequency (800 Hz) dielectric constant measurements were performed for a field strength of 10 v/cm with a RFT 1007 bridge. A nitrogen cryostat was used for sample thermal stabilization which is believed to be accurate to 0.01”. The temperature was measured to within 0.05” with the help of a copper-constantan thermocouple. The polarimetrical measurements employed the techniques and apparatus described in[8,9]. Dielectric hysteresis loops were obtained in the usual manner. For these measurements both fresh and one year old crystals were used.
INTRODUCTION
shift of the Curie point in NaH,(SeO,), crystals upon substitution of D for H was discovered by Blinc and co-workers[ l] and independently by Gavrilova-Podolskaya et a1.[2]. Such an isotope effect is entirely consistent with similar effects found in many other ferroelectrics. In addition to the shift of Curie temperature with deuteration, it was concluded that the character of the lower phase changed and that NaD,(SeO,), was antiferroelectric [ 1,3-61. As a similar isotope effect of a change in character of the lower phase has not yet been observed in any other system, one might question the conclusion that NaD,(SeO,), is antiferroelectric. In this connection a detailed analysis of the nature of the lowtemperature phase of NaD,(SeO,), was made. THE
LARGE isotopic
PREPARATION OF CRYSTALS AND EXPERIMENTAL PROCEDURE
The synthesis of NaD,(SeO& was carried out as follows, D,SeO, was obtained by dissolving SeO, in DzO; on interaction of D2Se0, with NqCO, solution in heavy water NaDa(SeO,), material was obtained. The crystals were grown in the temperature range 40-30” by the temperature decreasing method. During a month’s time colourless transparent crystals were grown, their weight reaching 50 g. The orientation of the orthogonal crystallophysic axes adopted
EXPERIMENTAL
RESULTS
1 shows the results of dielectric measurements on crystals for which the amount of deuteration was 60 and 98 per cent (deuteration per cent was determined in accordance with[3]). As one can see from the graph, the temperature of maximum dielectric constant, Ed, (which is considered for E= = 0 as the transition temperature) Figure
57
L. A. SHUVALOVet
No (H0.4D0.6)3 (SeO,),
J -00
PC)”
$0
-JUTeempera+ure
Fig. 1. Temperature dependence of dielectric constant components of Na(H0.4D0.B)S(SeO,), and NaD (SeO,), crystals at various biasing field strengths (EC = 17,). y- and z-component are shown for zero biasing field. Note that maximum of ey shifts in the field E, in the same direction as maximum of lz does.
shifts towards a higher temperature region with applied d.c. electric field, E= = E,. The character of the shift is nonlinear, it is closer to an lY3 law than to the linear one (Fig. 2.). Figure 2 also shows the dielectric constant, lZ, vs. d.c. biasing field for different temperatures in the low temperature phase. The behaviour of E, follows a Curie-Weiss law both for E = 0 and E # 0: for E = 0, C= (0.8&0*5) x103deg and T,=--1-l* O.YC, at T < Tt, = -5.65 O-l’%; C = (7*9* 1) X 103deg and T, =-34&4”C at T > T,, for NaD,(SeO,), (Fig. 3). These values satisfactorily agree with the results obtained by Blinc et al.[l]. It should be mentioned that the Curie-Weiss constant, CrcTtr, has a tendency to decrease with a d-c. biasing field
al.
increase. At the same time the characteristic slope change in the dependencies (E,- 1)-l (T) in the vicinity of the phase transition has a tendency to disappear with the d.c. field increase. But neither temperature hysteresis, nor discontinuity in dielectric constant change (as well as in other properties investigated) were observed within experimental error. To establish the symmetry of the lowtemperature phase of NaD,(SeO,), we used a polarimetrical method of measuring the rotation angle of the optical indicatrix, as was also the case with NaH,(SeO,), [7]. In NaD,(SeO,), crystal the indicatrix, at temperatures both above and below Tl,, rotates about the y-axis only (Fig. 4). This can be described in terms of the thermooptic effect. It is of interest to note that due to the phase transition the thermooptic coefficient changes its sign. The same phenomenon was observed in the case of NaH,(SeO,), although the existence of an interstitial phase makes the rotation curve more complicated [6]. We have found that it is impossible to observe domain structure by means of polarizing microscope method, while the domains in NaH,(SeO,), crystals are visible [7]. This fact is important for further discussion of the crystal symmetry. From the data reported by Blinc[ 1,3] one could expect that double hysteresis loops reflect the nature of NaD3(Se03)z itself. However our observations show that double loops take place only for aged specimens, while asgrown crystals show normal hysteresis loops. Double loops of our aged crystals have a low critical field comparable with the value of a coercive one which is about 2 kv/cm for -20°C. Cycling in the a.c. filed gradually decreases the critical field strength and leads to its complete disappearance, which is also in accordance with the behaviour of loops discussedin[l]. Temperature dependence of spontaneous polarization determined from dielectric hysteresis loops measurements is shown in Fig. 5. The dependence P, (T) is much
LOW-TEMPERATURE
PHASE OF NaDs (SeO&
59 6
dc. biasing
field
1W/cm)
Fig. 2. Shift of point of maximum of lz of NaD,(SeO& crystal under the action of an electric field and e&E,) dependence for different tempemtures in the low-temperature phase.
Temperature
(“C 1
Fig. 3. X-component of the reciprocal susceptibility vs. temperature for various biasing field strengths (E= = E,) .
smoother than that given by Blinc ef al. [ 11. The measurements of hysteresis loops were carried out at an a.c. field strength about two or three times the value of the coercive field. DISCUSSION
The questions under discussion are the character of transition, the nature of lowtemperature phase (i.e. the choice between
ferroelectric and antiferroelectric states), the symmetry of the phase and a comparison with NaH,(SeO,), (i.e. the peculiarity of isotopic effect). (1) As one can see from the results of the experiment described above the phase transition in NaD,(SeO,), has the features of both first and second order. This can be seen on the one hand, in the behaviour of the shift of
L. A. SHUVALOV
60
Temperature
etal.
PC)
Fig. 4. Thermal rotation of the optical indicatrix about the y-axis. dicatrix rotation about x and z axes does not take place.
dielectric constant maximum with the d.c. field, in the continuity of the physical properties during transition, in the absence within experimental error limits of thermal hysteresis of dielectric and optical properties and, on the other hand. in the change of slope of reci-
In-
procal susceptibility near the phase transition as well as in the difference between the extrapolated Curie-Weiss temperatures, T,, in the two phases. Thus one can conclude that we deal with either transition of second order, close to first order, or that of first order, close
No D, (SeOJr 6
0 -I20
-60 Temperature (“cl
-40
Fig. 5. Temperature dependence of spontaneous polarization for NaD, (SeO,), and Na(H0.4D0& (SeO,), crystals. P,,(T) curve for NaH,(SeO,), crystal according to[7].
LOW-TEMPERATURE
to second order. The choice between these two possibilities can be made only on the basis of more precise experiments. (2) Nevertheless the nature of the lowtemperature phase of NaD,(SeO,), can be determined whatever the order of transition may be. Thermodynamic theory predicts and experimental results confirm the shift of the point of maximum of e(T) with applied d.c. field towards the higher temperatures for ferroelectrics. So far there is no theoretically well-grounded answer to the question of the direction of shift of transition temperature for antiferroelectrics, however experiments show that the temperature of the maximum dielectric constant lowers under the external field [lo]. Therefore from our experiments on the shift of the temperature of the maximum dielectric constant one can suppose that NaD,(SeO,), is ferroelectric. This becomes all the more evident if one takes into account the fact that young crystals and crystals to which a field treatment has been given have normal hysteresis loops. The explanation of the fact that double hysteresis loops are observable should be based on consideration of the influence of impurities and ageing-rejuvenating effects rather than on the assumption that the antiferroelectric and field induced ferroelectric phase coexist. (3) The thermooptic measurements clearly demonstrate that NaD,(SeO,), exhibits a transition from the monoclinic phase 2/m to the monoclinic one. The impossibility to observe the domain structure by means of a polarizing microscope method also strongly confirms the fact that the crystal in the lowtemperature phase has a monoclinic symmetry. In addition, the electric measurements show that the P, vector lies in the mirror plane of the paraelectric phase (P, = P,,) whereas the y-component of P, is zero. All these measurements, as compared to the data reported in [7], clearly show that an interstitial triclinic ferroelectric phase of NaH,(SeO,), disappears upon a high degree of deuteration and
PHASE
OF NaDa (SeO&
61
NaD,(SeO,), undergoes a transition with 2/m + m symmetry change. From the analysis of previously known results and our own results obtained for NaD, (SeO& crystals we can make the following conclusions. i. The phase transition in NaD,(SeO,), has the features of both first and second order. More precise experiments are necessary to define transition order. ii. The transition is attended with the change of symmetry 2/m -+ m. An interstitial triclinic ferroelectric phase of NaH, (SeO,), crystals disappears upon a high degree of deuteration. iii. The low-temperature phase of NaD, (SeO,), crystals is ferroelectric not antiferroelectric as it has been considered up to the present. Acknowledgements-For Na(H,.,D,,.,), (SeO,), crystal the authors are indebted to K. S. Aleksandrov. We are very grateful to Dr. W. Reese for the useful criticism made upon reading the manuscript. REFERENCES R., JOVANOVIC A., LEVSTIC A., 1. BLINC and PRELESNIK A., .I. phys. Chem. Solids 26,1359 (1965). G. V., LUNDIN 2. GAVRILOVA-PODOLSKAYA A. G. and YUDIN A. L., Zh. Eksper. Teoret. Fiz. Pis’ma v redaktziyu 1,36 (1965). R. and VOVK D., Phys. Lett. 19, 177 3. BLINC
(1965). 4. BLINC R., POBERAJ S., SCHARA M and STEPISNIK J.,J. Phys. Chem. Solids 27,139l (1966). G. V., GABUDA 5. GAVRILOVA-PODOLSKAYA S. P. and LUNDIN A. G., Fiz. Tverd. Tela 9, 1166 (1967). ZHEREBTZOVA L. I. and ROSTUNTZEVA A. I., Kristaflogrqfiya 13,536 (1968). SHUVALOV L. A. and IVANOV N. R.. Phys.
Status Solidi 22,279 (1967). IVANOV
N. R. and SHUVALOV
L. A., Kristalio-
erafiva 11,614 (1966). iVANOV N. R., SCHUVALOV L. A., MIRENSKY A. V. and SHNYREV G. D.. Kristalloarafiva 12,307 (1967). 10. MAKITA Y., J. phys. Sot. Japan 20, 1567 (1965). 11. BLINC R. et al., Phys. Lett. 26A,290 (1968). L
__
APPENDIX The paper was presented at the 5’” Annual Solid State Physics Conference 3-6 January, 1968 held at the University of Manchester Institute of Science and Technol-
62
L. A. SHUVALOV
ogy, England. Some of the conclusions made in this paper are in agreement with the recent NMR data obtained by R. Blinc and co-workers[ll], which became available to us after manuscript had been sent to the editorial board. The results of our recent studies of the phase diagram of the system Na(H,_,D,),(Se03), show in addition to our previously obtained data that the interstitial triclinic ferroelectric phase exists in crystals at 0 < x < 0.5. The
et al.
upper phase transition curve is found to split into two curves, i.e. the sequence of transitions 2m + m + 1 + m occurs on lowering the temperature. (Zh. Eksper. Teorer. Fiz. Pis’ma u reduktziyu 8, 235 (1968)). Moreover, the more precise B (T) measurements show a small hysteresis on transition from paraelectric phase for all Na(H,_,D,), (SeO,), crystals which supports the suggestion on first order of the transition.
Pergamon Press 1969. Vol. 30, pp. 57-62.
Printed in Great Britain.
ON THE NATURE OF THE LOW-TEMPERATURE PHASE OF NaD,(SeO,), L. A. SHUVALOV, N. R. IVANOV, L. F. KIRPICHNIKOVA Institute of Crystallography
and N. M. SCHAGINA
of the Academy of Sciences of the USSR, Moscow, USSR
(Received 14 December 1967; in revisedform 3 May 1968) Abstract- Dielectric and optical properties of NaDJSeO& single crystals were studied. From the results obtained the low-temperature phase of NaD,(SeO,), is considered to be ferroelectric. It is shown that the phase transition is attended with 2/m + m symmetry change associated with disappearance on strong deuteration of the interstitial triclinic ferroelectric phase which was found to exist for NaH,(SeO,),.
by us coincides with the system of coordinates which we had used previously for NaH,(SeO& crystals [7]. This orientation differs from that developed by Blinc ei al.[4] so that our x-, y- and z-axes, being parallel to the main axes of the optical indica&ix, coincide with the directions -X cos 36”, Y, -2 cos 36” reported in [4]. Measurements were carried out using plates 0.5-l mm thick and 1 cm2 in area. Low-frequency (800 Hz) dielectric constant measurements were performed for a field strength of 10 v/cm with a RFT 1007 bridge. A nitrogen cryostat was used for sample thermal stabilization which is believed to be accurate to 0.01”. The temperature was measured to within 0.05” with the help of a copper-constantan thermocouple. The polarimetrical measurements employed the techniques and apparatus described in[8,9]. Dielectric hysteresis loops were obtained in the usual manner. For these measurements both fresh and one year old crystals were used.
INTRODUCTION
shift of the Curie point in NaH,(SeO,), crystals upon substitution of D for H was discovered by Blinc and co-workers[ l] and independently by Gavrilova-Podolskaya et a1.[2]. Such an isotope effect is entirely consistent with similar effects found in many other ferroelectrics. In addition to the shift of Curie temperature with deuteration, it was concluded that the character of the lower phase changed and that NaD,(SeO,), was antiferroelectric [ 1,3-61. As a similar isotope effect of a change in character of the lower phase has not yet been observed in any other system, one might question the conclusion that NaD,(SeO,), is antiferroelectric. In this connection a detailed analysis of the nature of the lowtemperature phase of NaD,(SeO,), was made. THE
LARGE isotopic
PREPARATION OF CRYSTALS AND EXPERIMENTAL PROCEDURE
The synthesis of NaD,(SeO& was carried out as follows, D,SeO, was obtained by dissolving SeO, in DzO; on interaction of D2Se0, with NqCO, solution in heavy water NaDa(SeO,), material was obtained. The crystals were grown in the temperature range 40-30” by the temperature decreasing method. During a month’s time colourless transparent crystals were grown, their weight reaching 50 g. The orientation of the orthogonal crystallophysic axes adopted
EXPERIMENTAL
RESULTS
1 shows the results of dielectric measurements on crystals for which the amount of deuteration was 60 and 98 per cent (deuteration per cent was determined in accordance with[3]). As one can see from the graph, the temperature of maximum dielectric constant, Ed, (which is considered for E= = 0 as the transition temperature) Figure
57
L. A. SHUVALOVet
No (H0.4D0.6)3 (SeO,),
J -00
PC)”
$0
-JUTeempera+ure
Fig. 1. Temperature dependence of dielectric constant components of Na(H0.4D0.B)S(SeO,), and NaD (SeO,), crystals at various biasing field strengths (EC = 17,). y- and z-component are shown for zero biasing field. Note that maximum of ey shifts in the field E, in the same direction as maximum of lz does.
shifts towards a higher temperature region with applied d.c. electric field, E= = E,. The character of the shift is nonlinear, it is closer to an lY3 law than to the linear one (Fig. 2.). Figure 2 also shows the dielectric constant, lZ, vs. d.c. biasing field for different temperatures in the low temperature phase. The behaviour of E, follows a Curie-Weiss law both for E = 0 and E # 0: for E = 0, C= (0.8&0*5) x103deg and T,=--1-l* O.YC, at T < Tt, = -5.65 O-l’%; C = (7*9* 1) X 103deg and T, =-34&4”C at T > T,, for NaD,(SeO,), (Fig. 3). These values satisfactorily agree with the results obtained by Blinc et al.[l]. It should be mentioned that the Curie-Weiss constant, CrcTtr, has a tendency to decrease with a d-c. biasing field
al.
increase. At the same time the characteristic slope change in the dependencies (E,- 1)-l (T) in the vicinity of the phase transition has a tendency to disappear with the d.c. field increase. But neither temperature hysteresis, nor discontinuity in dielectric constant change (as well as in other properties investigated) were observed within experimental error. To establish the symmetry of the lowtemperature phase of NaD,(SeO,), we used a polarimetrical method of measuring the rotation angle of the optical indicatrix, as was also the case with NaH,(SeO,), [7]. In NaD,(SeO,), crystal the indicatrix, at temperatures both above and below Tl,, rotates about the y-axis only (Fig. 4). This can be described in terms of the thermooptic effect. It is of interest to note that due to the phase transition the thermooptic coefficient changes its sign. The same phenomenon was observed in the case of NaH,(SeO,), although the existence of an interstitial phase makes the rotation curve more complicated [6]. We have found that it is impossible to observe domain structure by means of polarizing microscope method, while the domains in NaH,(SeO,), crystals are visible [7]. This fact is important for further discussion of the crystal symmetry. From the data reported by Blinc[ 1,3] one could expect that double hysteresis loops reflect the nature of NaD3(Se03)z itself. However our observations show that double loops take place only for aged specimens, while asgrown crystals show normal hysteresis loops. Double loops of our aged crystals have a low critical field comparable with the value of a coercive one which is about 2 kv/cm for -20°C. Cycling in the a.c. filed gradually decreases the critical field strength and leads to its complete disappearance, which is also in accordance with the behaviour of loops discussedin[l]. Temperature dependence of spontaneous polarization determined from dielectric hysteresis loops measurements is shown in Fig. 5. The dependence P, (T) is much
LOW-TEMPERATURE
PHASE OF NaDs (SeO&
59 6
dc. biasing
field
1W/cm)
Fig. 2. Shift of point of maximum of lz of NaD,(SeO& crystal under the action of an electric field and e&E,) dependence for different tempemtures in the low-temperature phase.
Temperature
(“C 1
Fig. 3. X-component of the reciprocal susceptibility vs. temperature for various biasing field strengths (E= = E,) .
smoother than that given by Blinc ef al. [ 11. The measurements of hysteresis loops were carried out at an a.c. field strength about two or three times the value of the coercive field. DISCUSSION
The questions under discussion are the character of transition, the nature of lowtemperature phase (i.e. the choice between
ferroelectric and antiferroelectric states), the symmetry of the phase and a comparison with NaH,(SeO,), (i.e. the peculiarity of isotopic effect). (1) As one can see from the results of the experiment described above the phase transition in NaD,(SeO,), has the features of both first and second order. This can be seen on the one hand, in the behaviour of the shift of
L. A. SHUVALOV
60
Temperature
etal.
PC)
Fig. 4. Thermal rotation of the optical indicatrix about the y-axis. dicatrix rotation about x and z axes does not take place.
dielectric constant maximum with the d.c. field, in the continuity of the physical properties during transition, in the absence within experimental error limits of thermal hysteresis of dielectric and optical properties and, on the other hand. in the change of slope of reci-
In-
procal susceptibility near the phase transition as well as in the difference between the extrapolated Curie-Weiss temperatures, T,, in the two phases. Thus one can conclude that we deal with either transition of second order, close to first order, or that of first order, close
No D, (SeOJr 6
0 -I20
-60 Temperature (“cl
-40
Fig. 5. Temperature dependence of spontaneous polarization for NaD, (SeO,), and Na(H0.4D0& (SeO,), crystals. P,,(T) curve for NaH,(SeO,), crystal according to[7].
LOW-TEMPERATURE
to second order. The choice between these two possibilities can be made only on the basis of more precise experiments. (2) Nevertheless the nature of the lowtemperature phase of NaD,(SeO,), can be determined whatever the order of transition may be. Thermodynamic theory predicts and experimental results confirm the shift of the point of maximum of e(T) with applied d.c. field towards the higher temperatures for ferroelectrics. So far there is no theoretically well-grounded answer to the question of the direction of shift of transition temperature for antiferroelectrics, however experiments show that the temperature of the maximum dielectric constant lowers under the external field [lo]. Therefore from our experiments on the shift of the temperature of the maximum dielectric constant one can suppose that NaD,(SeO,), is ferroelectric. This becomes all the more evident if one takes into account the fact that young crystals and crystals to which a field treatment has been given have normal hysteresis loops. The explanation of the fact that double hysteresis loops are observable should be based on consideration of the influence of impurities and ageing-rejuvenating effects rather than on the assumption that the antiferroelectric and field induced ferroelectric phase coexist. (3) The thermooptic measurements clearly demonstrate that NaD,(SeO,), exhibits a transition from the monoclinic phase 2/m to the monoclinic one. The impossibility to observe the domain structure by means of a polarizing microscope method also strongly confirms the fact that the crystal in the lowtemperature phase has a monoclinic symmetry. In addition, the electric measurements show that the P, vector lies in the mirror plane of the paraelectric phase (P, = P,,) whereas the y-component of P, is zero. All these measurements, as compared to the data reported in [7], clearly show that an interstitial triclinic ferroelectric phase of NaH,(SeO,), disappears upon a high degree of deuteration and
PHASE
OF NaDa (SeO&
61
NaD,(SeO,), undergoes a transition with 2/m + m symmetry change. From the analysis of previously known results and our own results obtained for NaD, (SeO& crystals we can make the following conclusions. i. The phase transition in NaD,(SeO,), has the features of both first and second order. More precise experiments are necessary to define transition order. ii. The transition is attended with the change of symmetry 2/m -+ m. An interstitial triclinic ferroelectric phase of NaH, (SeO,), crystals disappears upon a high degree of deuteration. iii. The low-temperature phase of NaD, (SeO,), crystals is ferroelectric not antiferroelectric as it has been considered up to the present. Acknowledgements-For Na(H,.,D,,.,), (SeO,), crystal the authors are indebted to K. S. Aleksandrov. We are very grateful to Dr. W. Reese for the useful criticism made upon reading the manuscript. REFERENCES R., JOVANOVIC A., LEVSTIC A., 1. BLINC and PRELESNIK A., .I. phys. Chem. Solids 26,1359 (1965). G. V., LUNDIN 2. GAVRILOVA-PODOLSKAYA A. G. and YUDIN A. L., Zh. Eksper. Teoret. Fiz. Pis’ma v redaktziyu 1,36 (1965). R. and VOVK D., Phys. Lett. 19, 177 3. BLINC
(1965). 4. BLINC R., POBERAJ S., SCHARA M and STEPISNIK J.,J. Phys. Chem. Solids 27,139l (1966). G. V., GABUDA 5. GAVRILOVA-PODOLSKAYA S. P. and LUNDIN A. G., Fiz. Tverd. Tela 9, 1166 (1967). ZHEREBTZOVA L. I. and ROSTUNTZEVA A. I., Kristaflogrqfiya 13,536 (1968). SHUVALOV L. A. and IVANOV N. R.. Phys.
Status Solidi 22,279 (1967). IVANOV
N. R. and SHUVALOV
L. A., Kristalio-
erafiva 11,614 (1966). iVANOV N. R., SCHUVALOV L. A., MIRENSKY A. V. and SHNYREV G. D.. Kristalloarafiva 12,307 (1967). 10. MAKITA Y., J. phys. Sot. Japan 20, 1567 (1965). 11. BLINC R. et al., Phys. Lett. 26A,290 (1968). L
__
APPENDIX The paper was presented at the 5’” Annual Solid State Physics Conference 3-6 January, 1968 held at the University of Manchester Institute of Science and Technol-
62
L. A. SHUVALOV
ogy, England. Some of the conclusions made in this paper are in agreement with the recent NMR data obtained by R. Blinc and co-workers[ll], which became available to us after manuscript had been sent to the editorial board. The results of our recent studies of the phase diagram of the system Na(H,_,D,),(Se03), show in addition to our previously obtained data that the interstitial triclinic ferroelectric phase exists in crystals at 0 < x < 0.5. The
et al.
upper phase transition curve is found to split into two curves, i.e. the sequence of transitions 2m + m + 1 + m occurs on lowering the temperature. (Zh. Eksper. Teorer. Fiz. Pis’ma u reduktziyu 8, 235 (1968)). Moreover, the more precise B (T) measurements show a small hysteresis on transition from paraelectric phase for all Na(H,_,D,), (SeO,), crystals which supports the suggestion on first order of the transition.