Influence of NH4-Rb substitution on the phase transitions with different kinds of proton disorder in mixed [(NH4)1- xRbx]3H(SO4)2 crystals Authors: A.I. Baranov, V.V. Dolbinina, E.D. Yakushkin, V.Yu. Vinnichenko, V. Hugo Schmidt, and S. Lanceros-Mendez This is an Accepted Manuscript of an article published in Ferroelectrics on [date of publication], available online: http://www.tandfonline.com/10.1080/00150199808015049. A.I. Baranov, V.V. Dolbinina, E.D. Yakushkin, V.Yu. Vinnichenko, V.H. Schmidt, and S. Lanceros-Mendez, “Influence of NH4-Rb substitution on the phase transitions with different kinds of proton disorder in mixed [(NH4)1-xRbx]3H(SO4)2 crystals,” Ferroelectrics 217, 285-295 (1998). http://dx.doi.org/10.1080/00150199808015049 Made available through Montana State University’s ScholarWorks scholarworks.montana.edu Ferroelecrrics, 1998, Vol. 217, pp. 285-295 Reprints available directly from the publisher Photocopying permitted by license only 0 1998 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia. INFLUENCE OF NH4-Rb SUBSTITUTION ON THE PHASE TRANSITIONS WITH DIFFERENT KINDS OF PROTON DISORDER IN MIXED [(NH,& --xRbx]3H(S04)2 CRYSTALS A. I. BARANOV",*, V. V. DOLBININA", E. D. YAKUSHKIN", V. Yu. VINNICHENKO ", V. H. SCHMIDT and S . LANCEROS-MENDEZ a Institute of Crystallography of RAS, Moscow, I 1 7333, Russia; Phys. Dept., Montana State University, Bozeman, M T 59717, USA (Received 24 March 1998: In$nalform 12 May 1998) The role of ammonium ions in order - disorder ferroelastic, ferroelectric and structural phase transitions was studied in mixed [(NH4)1_xRbx]3H(S04)2 0 < x < 1 crystals by dielectric spectroscopy and ac calorimetry. A new superprotonic phase was detected in compounds with x > 0.6. It was found that small Rb concentrations, x < 0.05, strongly smear out the temperature anomalies of dielectric constant and specific heat for phase transitions I1 - 111, I11 - IV, IV- V and V-VIII. At x>O.O9 these phase transitions are fully suppressed. Finally, for Rb concentrations 0.05 < x < 0.9 a proton glass phase appears below 35 K. Keywords: Glass states; ferroelectrics; ferroelastics; proton disorder INTRODUCTION It is well known that in most ferro/antiferroelectric ammonium containing compounds the NH: ions strongly influence the ferroelectric and other physical properties. For example, in crystals of the KDP family the substitution of alkali ions by NH; ions changes the ferroelectric to antiferroelectric ordering."] In some compounds like M2S04 or M (M = K, Rb, NH4, Cs) the phase diagrams for the alkali metal salts are much simpler *Corresponding author. e-mail: root@theory.incr.msk.su 285 286 A. I. BARANOV et al. than for the isomorphous ammonium salts.[2* 31 Such differences are displayed most strictly in crystals of the M3H(A04)2 family where M = K , Rb, NH4, Cs and A=S, Se.[4p81 In particular, (NH4)3H(S04.)2 (TAHS) exhibits five successive phase transitions[5' 'I I t 4 1 3 K -+ I1 + 265 K -+ I11 +- 1 3 9 K c IV -+ 133 K + V -+ 63 K + VII R j / m A2/a P2/n ? ? 'i' while Rb3H(S04)2 (TRHS) has symmetry isomorphous to monoclinic phase I1 of TAHS and undergoes no defined phase transitions from the melting point down to 4 K.[63 81 At room temperature TAHS is monoclinic with space group C;,,(A~I~).['. ''1 Its crystal structure consists of two structurally inequivalent NH: ions and SO:- groups. NH: (1) ions occupy a general position and form pure NH,f layers while NH,f (2) ions occupy a special position on the two-fold axis and form mixed NH:, SO:- layers. The two adjacent SO:- groups are linked by strong symmetrical (at room temperature) hydrogen bonds with length 2.540 A. These acid hydrogen bonds being isolated from each other clo not form any network through the crystal. At room temperature the TRHS crystal has the same structure and symmetry space group I1 However the length of acid H-bonds in TRHS (2.486 A) is shorter than in TAHS. The succession of the low temperature phase transitions in TAHS is very sensitive to isotopic H- D substitutionL8' 12] and hydrostatic pre~sure."~] Moreover, the first of mixed [(NH4)0.14 Rb0,86]3H(S04)2 crystals points out that at low temperatures the compounds with x>O.1 reveal transitions into a glass phase instead of a ferroelectric phase transition V- VII. These facts suggest that ammonium ions owing to additional rotational degrees of freedom and weak N-H . . . O hydrogen bonds strongly increase lability of the crystalline lattice of TAHS. Accordingly, mixed [(NH4)1-,RbX]3H(S04)2 (TRAHS) crystals are considered as good model substances to clarify the special role of ammonium ions in the peculiar properties of ammonium salts. In the present work the influence of NH4-Rb substitution on the phase transitions and some properties of the mixed crystals [(NH4), -xMx]3 H(SO4)2 have been studied. EXPERIMENTAL Mixed [(NH4)1-,Rb,]3H(S04)2 single crystals with x = 0, 0.03, 0.05, 0.09, 0.14, 0.18, 0.25, 0.41, 0.50, 0.71, 0.85 and 1.0 were grown from water NH4-Rb SUBSTITUTION IN MIXED [(NH4)I _,Rbx]3H(S04)2 287 solutions by the slow evaporation method. The concentration of the components in solid solutions was determined by direct chemical analysis. The concentrations of Rb3H(S0& in the crystals versus those in the saturated solutions are shown in Figure 1. An autobalanced Quad Tech 7600 Precision RLC Meter and Tesla BM- 43 1-E bridge were used for measurements of the complex dielectric constant in the frequency range 10Hz-200MHz. A closed cycle helium cryostat (Janis Research Co., Inc. Model C210) and home built thermostat were used in the temperature ranges 10 - 320 K and 300 - 600 K, respectively. The samples had the form of rectangular plates with size 0.5 x 0.5 x 0.1 cm3 oriented perpendicular to the pseudo-hexagonal axis. Silver paint was used for electrodes. The standard method of admittance plots was used to calculate dc bulk conductivity. 1.0 0.8 g E E 0.6 - ([I * 0 I= 0.4 a R i .- 0.2 0.0 8' 0' I I I I 0.0 0.2 0.4 0.6 0.8 1 .o Rb in solution, mol% 1 .o 0.8 0.6 0.4 0.2 0.0 FIGURE 1 water solutions. Rb content in [(NH&,RbX]3H(S04)2 crystals versus Rb content in saturated 288 A. I. BARANOV et al. Specific heat measurements were carried out by the ac-calorimetric technique in quasi-static regime with heating/cooling rate of 0.02 deg/min. Thin plates of single crystals with dimensions of 2 x 2 x 0.1 mm3 were used as samples. RESULTS 1. (NH4)JH(S04)2 Crystal The results of previous study of anisotropy of dielectric properties of (NH4)3H(A04)2 and Rb3H(AO4),rS3 6 , 12, 14] suggest that below the ferroe- lastic phase transition 1-11 these crystals can be considered as electrically biaxial with the predominant direction of the polarization along the pseudotrigonal c axis. Therefore in the present study the measurements of dielectric properties and conductivity of [(NH4)l-,Rb,]3H(S04)2 were carried out on plates oriented perpendicular to the c-axis. Pure TAHS shows well-defined anomalies of dielectric constant and specific heat corresponding to five successive phase transitions (Figs. 1 and 2) . Recently it was shown['51 that the high-temperature phase transition I - I1 is improper ferroelastic and superprotonic simultaneously. Increasing sym- metry from monoclinic (A2/a) to trigonal (R?/rn) is accompanied by dynamical disordering of the acid hydrogen bond network which leads to a high protonic conductivity in Phase I (Fig. 3). The 1-11 phase transition entropy AS= 2.5 f 0.2 cal . mol-' . deg-'[16' is close to the configurational entropy AS= Rln3 = 2.19 cal . mol-' . deg-' for this kind of order-disorder phase tran~ition."~] The quasi-two-dimensional character of conductivity cra E >> crc agrees well with the two-dimensional geometry of the disordered acid hydrogen bond network.[I7] The phase transition 11-111 is also of the order-disorder type. However, the entropy AS= 2.2 cal . mol-' . deg-' of this transition is much larger than the configurational entropy associated only with proton ordering in the acid H-bonds double-well potentials (AS= 1.3 cal . mo1-I . deg-I). Moreover, its temperature is not changed by deuteration. According to results of X-ray study"'] C-centered lattice disappears in Phase I11 and the acid hydrogen bonds become asymmetric. Hence, the phase transition I1 - I11 can be triggered by orientational ordering of NH,f (1) ions and accompanied by partial ordering of acid protons in asymmetric hydrogen bonds. A specific feature of low temperature phases of TAHS is a ferroelectric soft mode which exists in a wide temperature range including phases I1 -V. NH4-Rb SUBSTITUTION IN MIXED [(NH4)1~xRb~]3H(S04)2 289 P 4 -*-TAHS 150 MHz -0-TAHS 1 MHz 60 - - TRHS 150 MHz -+-TRHS 1 MHz ~ ' 1 ' 1 ' ~ 1 ~ ' ' ' ~ ' ~ 1 ~ ' 1 ' ' T,K 0 50 100 150 200 250 300 350 400 450 500 FIGURE 2 Temperature dependences of dielectric constant E, in (NH4),H(S0& and RW(S04)z. The temperature dependence of the soft mode is reflected in the Curie- Weiss behavior of the E,(T) dependence over all these phases (Fig. 2). The values of Curie constants C are about lo3 K (CII = 2.9.103 K, CI1l = 1.4. lo3 K, C ~ V x Cv x 0.6. lo3 K) and are typical for proper order - disorder ferroelectric or antiferroelectric transitions. The smoothed max- imum on dependence of E,(T) does not correspond to any structural phase transition and could be assigned to a trace of over-critical phase transition."81 The phase transition I11 - IV is of second order, while IV -V is of first order. These transitions are accompanied by small entropy changes, which suggests small differences of degree of ammonium and acid proton ordering in phases 111, IV and V. Hence the Curie-like behavior of E, in Phases IV and V is indicative of antiferroelectric ordering below TIII-IV, although double hysteresis loops were not observed in these phases. 290 A. I . BARANOV et al. 2 0 -2 E b c -I v -4 -6 -8 1 ' 1 - 1 ' 1 ' i -0- x= 1 .oo if- ~ ~ 0 . 7 1 4 x=0.51 --A- x=o 1.8~10.~ 2.0~10.~ ~ . Z X ~ O - ~ 2 . 4 ~ 1 0 - ~ 2 . 6 ~ 1 0 - ~ 2 . 8 ~ 1 0 ' ~ Ill, K ' FIGURE 3 Arrhenius plot of bulk dc conductivity 00 in [(NH4),-xRb,]3H(S0& crystals measured along the c axis. Our results show a step-like increase of E, at the first order phase transition V-VII which we attribute to domain dielectric dispersion in ferroelectric Phase VII (Fig. 2). The dielectric constant E, of a single domain sample calculated from frequency dependencies of complex dielectric constant in frequency range 10 Hz- 1 MHz shows quite different behavior from that measured at 1 MHz. Thus, in the vicinity of the ferroelectric transition V-VII the dependence of E,(T) does not obey the Curie-Weiss law either above nor below TV-VII. Therefore this phase transition can be interpreted as a transition between two ordered phases: antiferroelectric and ferroelectric. 2. RbSH(SO& Crystal According to results of previous studies,['] TRHS exhibits two irreversible structural phase transitions at 329 and 399K. Our experiments do not NH4-Rb SUBSTITUTION IN MIXED [(NH4)1-,RbX]3H(S0& 29 1 confirm the existence of these phase transitions. However, a well defined reversible first order phase transition was detected at temperature T I , ~ I I = 476K which is 50K lower than the melting point. In the vicinity of this transition the anomaly of temperature dependence of protonic conductivity is quite similar to that observed at the superprotonic phase transition 1-11 in TAHS (Fig. 3). Therefore, we can conclude that high- temperature phase I’ in TRHS is also superprotonic. Disappearing of anisotropy of dc conductivity and high-frequency dielectric constant in phase I’ is consistent with cubic symmetry, but a cubic phase for such a complex material seems unlikely and would have to be confirmed by X-ray diffraction. In TAHS, the superprotonic paraelastic phase I is trigonal. Below T1t-11, E, increases with decreasing temperature in accord with the Curie- Weiss law. The value of Curie constant C = 1.5. lo3 K is close to that obtained by Gesi in earlier dielectric experimentsr6] where also it was shown that no phase transition is observed in TRHS down to 4.2K. 3. Mixed [(NH4)I-xRbx]s H(S04)2 Crystals At low Rb concentrations (xx>O.O5, the monoclinic 292 x=o.o9 A. I. BARANOV et al. - 0.1 - i'"I-'" v @--e . *-a* . I 0.0 - m. .*,* 0.1 0.0 20 80 - 60 40 20 0 x=o r 1 6o 0.1 0.0 . . I I 10 FIGURE 4 Temperature dependences of dielectric constant E, measured at 1 MHz (solid line) and excess part of specific heat AC, (open circles) in [(NH4)l-,Rbx]3H(S04)2 crystals. symmetry (A2/a) persists from the superprotonic phase transitions down to OK (Fig. 6 ) . The concentrations xx0.05 and x x O . 9 correspond to the critical upper and lower concentrations for the glass regime which exists at low temperatures. I I I - 0 w (4 I i i 8 ' 8 20 40 60 80 100 0.4 0.3 - - 0 w 0.2 0.1 00 (b) 2.0 00 20 40 60 80 100 T, K FIGURE 5 Temperature dependences of real (a) and imaginary (b) part of complex dielectric constant E: measured at 1 MHz in [(NH4)1-rRbr]3H(S04)2 crystals. 294 A. I. BARANOV et al. 550 I 1 I I _ - _ - - - - _ - _ _ - - - 500 - 400 - 350 1 Y 200 150 - - - 100 v 50 I,, I I I I I I I I I I I ..... 120 ’“P- l o o ] II *o/ 60 4 40- 20 - VII 000 005 010 015 020 Glass phase - I ’ I I I I , I 0.0 0.2 0.4 0.6 0.8 1 .o Rb concentration x FIGURE 6 x, T phase diagram of [(NH4)I-xRbx]3H(S04)2 determined from dielectric, conductivity and calorimetric data. CONCLUSION Dielectric and calorimetric measurements of our newly investigated mixed [(NH4)I-,Rb,]3H(S04)2 (0 5 x 5 1) crystals have been performed in th.e temperature range 18 - 600 K. A new high-temperature superprotonic phase was detected for compositions with x > 0.6. 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