Crystallography Reports, Vol. 48, No. 6, 2003, pp. 1012–1037. Translated from Kristallografiya, Vol. 48, No. 6, 2003, pp. 1081–1107. Original Russian Text Copyright © 2003 by Baranov. PHYSICAL PROPERTIES OF CRYSTALS Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences Crystals with Disordered Hydrogen-Bond Networks and Superprotonic Conductivity. Review A. I. Baranov Shubnikov Institute of Crystallography, Russian Academy of Sciences, LeninskiÏ pr. 59, Moscow, 119333 Russia e-mail: [email protected] Received June 11, 2003 Abstract—The class of hydrogen-containing salts in which the phases with dynamically disordered hydrogen- bond nets are formed has been considered. Unlike other hydrogen-bonded crystals, the crystals of this class are characterized by delocalized hydrogen bonds producing considerable influence on their physical and physical– chemical properties. The structural transitions between the phases with ordered and disordered hydrogen-bond networks are described with the emphasis being made on the structural mechanisms of anomalously high (superprotonic) conductivity associated with delocalization of hydrogen bonds. The perspectives of the use of crystals with delocalized hydrogen bonds in fuel cells and other electrochemical devices are discussed. © 2003 MAIK “Nauka/Interperiodica”. LIST OF CONTENTS INTRODUCTION Introduction It is well known that hydrogen in crystals has spe- 1. Geometry of Hydrogen-Bond Networks and Pro- cific crystallochemical parameters and interacts with ton Disorder the surrounding atoms forming specific hydrogen 2. Protonic Conductivity and Proton Defects in Crys- bonds [1]. The formation of hydrogen bonds consider- tals with Ordered Hydrogen-Bond Networks ably influences the structure of crystals, their physical 3. Protonic Conductivity and Proton Dynamics in and physicochemical properties, and also the thermo- Crystals with Dynamically Disordered Hydrogen- dynamic stability of the crystalline state. Since hydro- Bond Networks gen bonds (H bonds) are directional, their arrangement 4. Proton Transfer and Energy of Hydrogen-Bond in crystals is usually characterized by networks of dif- Formation ferent geometries, which depend on the structural and 5. Effect of Hydrostatic Pressure on Protonic Con- chemical characteristics of each compound. The ductivity. Activation Volume of Protons dimension and orientation of networks of hydrogen bonds and also the characteristics of the proton dynam- 6. Proton Dynamics in Phases with Disordered ics on an H bond determine the physical properties of Hydrogen-Bond Networks crystals such as piezoelectric effect, spontaneous polar- 7. Superprotonic Phase Transitions ization, dielectric nonlinearity, and protonic conductiv- 8. Characteristics of Superprotonic Phase Transitions ity [2, 3]. in Hydrate Phases of Crystals with Disordered Hydrogen-Bond Networks Most of the known crystals have localized hydrogen 9. Effect of Hydrostatic Pressure on Superprotonic bonds; i.e., the positions and directions of hydrogen Phase Transitions and Thermodynamic Stability of bonds are fixed by the crystal symmetry. However, Superprotonic Phases comparatively recently, crystals with delocalized H bonds have also been found [4–14]. The characteristic 10. Effect of Cationic and Anionic Substitution on feature of these crystals is a disordered sublattice of Superprotonic Phase Transitions and Protonic hydrogen atoms; i.e., the number of structurally and, Conductivity therefore, energetically equivalent positions of hydro- 11. Isotopic Effects in H–D Substitutions gen atoms forming H bonds exceeds the number of 12. Practical Applications of Superprotonic Crystals hydrogen atoms in the unit cell. The most important and Synthesis of New Protonic Solid Electrolytes characteristics of crystals with delocalized bonds or Conclusions crystals with disordered hydrogen-bond networks 1063-7745/03/4806-1012 $24.00 © 2003 MAIK “Nauka/Interperiodica” CRYSTALS WITH DISORDERED HYDROGEN-BOND NETWORKS 1013 Table 1. Phase-transition temperatures Tsp, activation enthalpies in phases with DHBNs, and lengths of hydrogen bonds in superprotonic crystals Change of symmetry Length of H bond, Crystal T* , K H , eV sp in superionic transition Å a 2.530 RbHSO 466**) [111] B21/a tetragonal [111] 0.32 4 2.620 2.514 (NH) HSO 496***) [112] B21/a tetragonal [112] 0.32 4 4 2.620 CsHSO4 414 [4] P21/c I41/amd [ 21] 2.57 0.26 CsDSO4 409 [5] P21/c I41/amd [ 21] 2.638 0.27 2.53 RbHSeO 470 [6, 8] I2 tetragonal [6, 8] 0.25 4 2.55 2.52 (NH) HSeO 428 [6, 8] I2 tetragonal [6, 8] 0.25 4 4 2.56 CsHSeO4 384 [4, 94] P21/c I41/amd [21, 81] 2.57 0.25 K3H(SO4)2 463 [134] A2/a ? [134] 2.49 Rb3H(SO4)2 475 [121, 128] C2/c cubic [128] 2.486 (NH4)3H(SO4)2 413 [80] A2/a R3m [121] 2.540 0.23 [(NH4)0.3Rb0.7]3H(SO4)2 463 [128] A2/a R3m [121] 495 [128] R3m cubic [128] K3H(SeO4)2 393 [135] A2/a R3m [135] 2.524 Rb3H(SeO4)2 455 [9] A2/a R3m [9] 2.541 0.26 (NH4)3H(SeO4)2 305 [10, 11] A2/a R3m [122] 2.486 0.27 Cs3H(SeO4)2 451 [10, 11, 136] A2/a R3m [136] 2.71 0.37 Cs3D(SeO4)2 448 [136] A2/a R3m [136] 0.39 (NH4)4H3(SeO4)3 378 [10.11] A2/a R3m [11] 0.11 Cs5H3(SO4)4 · xH2O P63/mmc [23] 0.66 Cs5(H,D)3(SO4)4 · x(H,D)2O P63/mmc [23] 0.66 Cs5H3(SeO4)4 · H2O 346 [35, 94] Pbcn P63/mmc [24] 0.63 K9H7(SO4)8 · H2O 393 [95] P2/c ? [95] CsH2PO4 503 [12, 120] P21/c Pm3m [133] 2.48 0.32 β -Cs3H(SO4)2[Hx(P,S)O4] 398 [127] C2/c ? [127] α -Cs3H(SO4)2[H2(PO4] 390 [126] P21/n ? [126] CsH2AsO4 435 [12] P4/ cubic [12] 0.3 Cs1.5Li1.5H3(SeO4)2 [125] 1.0 * For first-order phase transitions the temperature Tsp correspond to heating. ** Hydrostatic-pressure-induced phase transition, p = 0.6 GPa. *** Hydrostatic-pressure-induced phase transition, p = 0.28 GPa. Ð7 2 Ð1 (DHBNs) are fast proton diffusion, Dp ~ 10 cm s , present, a rather large number of groups of crystals pos- and high protonic conductivity, σ ~ 10Ð2 ΩÐ1 cmÐ1 [4–8]. sessing phases with DHBNs are known (Table 1). These are the crystals described by the general formu- The present review is dedicated to the structural characteristics of crystals with dynamically disordered las MeHAO4, Me3H(AO4)2, Me4H2(AO4)3, networks of H bonds and their effect on the physical Me5H3(AO4)4 · xH2O, Me9H7(AO4)8 · H2O (Me = K, Rb, and physicochemical properties of these crystals. At NH4, Cs; A = S, Se), and CsH2BO4 (B = P, As) and the CRYSTALLOGRAPHY REPORTS Vol. 48 No. 6 2003 1014 BARANOV solid solutions on their basis. At present, we have vast atoms. Therefore, the H bonds in these structures are experimental and theoretical material waiting for gen- positionally and orientationally disordered. If this dis- eralization. In addition to the fundamental aspect— order is dynamical, then protons and, therefore, hydro- deepening of our knowledge on the nature of H bonds gen bonds are delocalized. It should be indicated that, and their effect on the properties of the crystalline sub- in principle, this characteristic of hydrogen bonds dif- stances—crystals with DHBNs are also of great interest fers from the well-known bifurcated three-center as promising materials for practical applications. hydrogen bonds whose formation is associated with Because of high protonic conductivity, these crystals deficit of protons [1, 18]. This type of hydrogen bonds are actively studied as possible protonic solid electro- is beyond the scope of the present review. lytes for the membranes of fuel cells (electrochemical generators of current) [15–17]. The difference between ordered and disordered hydrogen-bond networks is illustrated by Fig. 1, which shows the structures of monoclinic (zero-dimensional 1. GEOMETRY OF HYDROGEN-BOND ordered network with localized hydrogen bonds) and NETWORKS AND PROTON DISORDER trigonal (two-dimensional disordered network with dis- ordered hydrogen bonds) phases of the crystals In a majority of crystals, hydrogen atoms forming H described by the general formula Me H(AO ) . bonds completely occupy one or several crystallo- 3 4 2 graphic positions in the crystal structure. In other words, the number of structurally equivalent lattice 2. PROTONIC CONDUCTIVITY AND PROTON sites corresponding to the total multiplicity of the crys- DEFECTS IN CRYSTALS WITH ORDERED tallographic positions gp energetically most favorable HYDROGEN-BOND NETWORKS for the formation of H bonds is equal to the number of hydrogen atoms per unit cell, np. This signifies that the The experimental data show [13, 14, 19] that the centers of hydrogen bonds in these crystal structures dimension and geometry of H-bond networks produce are ordered and the hydrogen bonds, being directional, no effect on anisotropy of protonic conductivity in crys- form an ordered hydrogen-bond network (OHBN). The tals with OHBNs. Thus, protonic conductivity in the notion of a hydrogen-bond center is introduced because monoclinic phase of a CsH2PO4 crystal with the one- the proton position usually does not coincide with the dimensional network of hydrogen bonds is almost iso- hydrogen-bond center. As a rule, the latter is character- tropic (Fig. 2). The protonic conductivity in the mono- ized by a potential with two minima and two proton clinic phases of the Me3H(AO4)2 crystals with the zero- positions (not necessarily structurally equivalent). dimensional network of H bonds is also three-dimen- Therefore, it is important to distinguish between proton sional but strongly anisotropic (Fig. 3). These facts disorder in a two-minimum potential of an H bond and indicate that proton migration (diffusion) proceeds not disorder in the positions of H bonds. only over conventional proton sites but also over inter- Networks of H bonds in crystals are usually charac- stitials.