The replacements in the cation sublattice in superprotonic crystals

Abstract

A fundamental problem of modern condensed state physics is the relationship between structures and physical properties of the crystals. To elucidate the effect of isomorphic substitution on the kinetics of phase transitions, single crystals of (K1х(NH4)x)mHn(SO4)(m+n)/2•yH2O solid solutions are grown in the K3H(SO4)2-(NH4)3H(SO4)2-H2O system which end members are known to undergo superprotonic phase transitions with essentially different kinetics. The use of K3H(SO4)2 and (NH4)3H(SO4)2 solutions with a ratio of 9:1 led to a growth of crystals of the same structure type as K3H(SO4)2: the single crystals were described by the chemical formula (K1-x(NH4)x)3H(SO4)2 [1]. Based on the refinement of the structural model, it was concluded that the (K1-x(NH4)x))3H(SO4)2 samples under study contain no less than 3% ammonium. This amount of ammonium turned out to be sufficient to significantly change the kinetics of structural phase transitions in comparison with K3H(SO4)2 crystals, which is related to the formation of additional hydrogen bonds and the change in the anisotropy of the coordination environment of both cations and SO4 tetrahedra. The use of K3H(SO4)2 and (NH4)3H(SO4)2 solutions with a ratio of 3:7 led to growth of single crystals of the same structure type as the high-temperature phase of (NH4)3H(SO4)2 [2]. The refinement of the structure with allowance for the replacement of potassium with ammonium and the analysis and comparison of the main crystallographic characteristics with the K3H(SO4)2 crystal revealed that the (K1-x(NH4)x)3H(SO4)2 crystal under study contains about 57% ammonium. Studies of dielectric properties revealed that the grown samples have high protonic conductivity at the room temperature. Solutions of K3H(SO4)2 and (NH4)3H(SO4)2 with different ratio (K:NH4) crystallize in different structure types. Using K3H(SO4)2 and (NH4)3H(SO4)2 solutions with a K:NH4 ratio close to 1:1, one can grow single crystals with a structure type differing from that of the initial compounds: the chemical formula of the grown single crystals is (K1x(NH4)x)9H7(SO4)8⋅H2O [3]. The structural study of the grown samples revealed that they contain no less than 4% ammonium. A comparison of the data on the K9H7(SO4)8⋅H2O and (K1-x(NH4)x)9H7(SO4)8⋅H2O crystals [3] showed that the replacement of potassium with ammonium reduced the structural PT temperature by 8 K. At the same time, the formation of additional hydrogen bonds of NH4 groups, which block the conductivity channels for K ions, led to a decrease in conductivity by 4 orders of magnitude, which indicates simultaneously the contribution of K ions to the conductivity of (K,NH4)9H7(SO4)8⋅H2O compound.

Conductivity, its anisotropy and changes as a manifestation of the features of the atomic and real structures of superprotonic [K1–x (NH4) x ]3H(SO4)2 crystals

“…No significant differences in the geometry of hydrogen bonds of N1H 4 groups (N1, H4 and H5 atoms) in crystals [K 0.20 (NH 4 ) 0.80 ] 3 H(SO 4 ) 2 and [K 0.43 (NH 4 ) 0.57 ] 3 H(SO 4 ) 2 have been detected. The difference electron-density maps are similar to those given by Selezneva et al (2017) for [K 0.43 (NH 4 ) 0.57 ] 3 H(SO 4 ) 2 compounds and, therefore, they are not given here. The position of H4 atom was determined from the calculated electron-density distribution.…”

“…Structural analysis X-ray diffraction studies of [K 1-x (NH 4 ) x ] 3 H(SO 4 ) 2 single crystals grown from K 3 H(SO 4 ) 2 and (NH 4 ) 3 H(SO 4 ) 2 solutions with a ratio of 1:9 were performed. Our study shows that these crystals have trigonal symmetry (Table 1) as well as [K 0.43 (NH 4 ) 0.57 ] 3 H(SO 4 ) 2 crystals grown from solutions with a ratio of 3:7 (Selezneva et al, 2017). For comparison, the data of [K 0.43 (NH 4 ) 0.57 ] 3 H(SO 4 ) 2 crystals are also given in Table 1.…”

“…Based on this, the refinements of structure models of [K 1-x (NH 4 ) x ] 3 H(SO 4 ) 2 were carried out. The coordinates of the basic atoms from Selezneva et al (2017) were used as the initial data to refine the structural parameters. The position of hydrogen atom H1 was determined from the difference electron-density maps.…”

“…This work is a continuation of the studies of superprotonic crystals obtained in the K 3 H(SO 4 ) 2 -(NH 4 ) 3 H(SO 4 ) 2 -H 2 O water-salt system (Dmitricheva et al, 2014;Selezneva et al, 2017). Superprotonic crystals belong to the family with the general formula M m H n (AO 4 ) (m+n)/2 ÁyH 2 O (M = K, Rb, Cs, NH 4 ; AO 4 = SO 4 , SeO 4 , HPO 4 , HAsO 4 ).…”

Исследование фазовых переходов в кристаллах (Cs,NH-=SUB=-4-=/SUB=-)-=SUB=-4-=/SUB=-(HSO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-(H-=SUB=-2-=/SUB=-PO-=SUB=-4-=/SUB=-)

“…С точки зрения исследований протонного транспорта представляет интерес изучение возможности модификации систем водородных связей. Примером таких исследований являются работы по изучению соединений с замещением, например, (K,NH 4 ) 3 H(SO 4 ) 2 [6,7]…”