Effect of deuteration on the electrical conductivity, dielectric constant and phase transitions in LiNH4SO4

Abstract

Measurements of dc conductivity and dielectric constant show that deuteration causes an upward shift of the high temperature phase b'ansition point from 186.5 to 191°C and a downward shift of the low temperature transition point from 10 to -1.5°C in LiNa.S04. Mechanisms of phase transitions and of electrical transport in the crystal are discussed. In this letter we present the fust report on the effect of deuteration on the electrical conductivity, dielectric constant and phase transitions in lithium ammonium sulphate (LAS) which is an extremely interesting room temperature ferroelectric material exhibiting two phase transitions of the fust order at atmospheric pressure with symmetry changes: phase I (Pnam) (paraelectric) 186.5°C I phase II (Pna2 1 ) (ferroelectric) l[S phase III (P2./a) (ferroelastic) [1][2][3][4][5][6][7][8][9]. Recent investigations on this material include the NMR studies at high pressure [10,11]. the electrical conductivity {12,13] and ionic thermo-current (14 J measurements.'e -'0 u 'e -H Solution grown single crystals of LiND 4 S0 4 (DAS) were prepared from high purity LAS by repeated recrystalUsation using heavy water of isotopic purity 99.8%. The details of sample preparation and measurement of conductivity made under vacuum (10-3 Torr) conditions have been reported earlier [12]. The dielectric measurements were carried out under the same vacuum conditions using a direct reading LC meter having a prOvision to annul the effect of lead and fringe capacitances which in the present case were estimated accurately [15 J. The LC meter readings were counter checked by a Marconi Universal bridge The rates oftemperature variations used were l°C/h in the vicinity of the transition points and SOC/h in the other regions.

Order–Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3] (M = Ni, Mn)

“…However, there are some instances where T c is weakly affected by deuteration. For instance, in LiNH 4 SO 4 for which the phase transition mechanism is associated mainly with transition into a state of free rotation of NH 4 + ions at high temperatures, the T c increased only by 4.5 K upon deuteration . On the other hand, for purely order–disorder phase transition in KNO 3 , the 15 N-enriched KNO 3 showed a slight decrease of T c when compared to the natural 14 N isotope .…”

“…For instance, in LiNH 4 SO 4 for which the phase transition mechanism is associated mainly with transition into a state of free rotation of NH 4 + ions at high temperatures, the T c increased only by 4.5 K upon deuteration. 13 On the other hand, for purely order−disorder phase transition in KNO 3 , the 15 N-enriched KNO 3 showed a slight decrease of T c when compared to the natural 14 N isotope. 14 The very weak H-atom isotopic effect for DMMn and negative effect for DMNi exclude the possibility of proton (deuteron) movement along N−H···O (N−D···O) bonds as the microscopic origin of the first-order phase transition in these MOF.…”

On Phase Transitions in LiNH4SO4

“…According to [16] xyyY(T,) is even higher than xyv(T1); this feature could be possibly explained within our model when setting appropriately the T-axis in the phase diagram essentially due to the fact that negative b, may reduce significantly the denominator of (14).…”