Synthesis, crystal structure and properties of alluaudite-like triple molybdate Na sub(25)Cs sub(8)Fe sub(5)(MoO sub(4)) sub(24 )

A new triple molybdate Na sub(25)Cs sub(8)Fe sub(5)(MoO sub(4)) sub(24 ) was synthesized using solid state reactions and studied with X-ray powder diffraction, second harmonic generation (SHG) technique, differential scanning calorimetry, Mossbauer and dielectric impedance spectroscopy. Single crystals of Na sub(25)Cs sub(8)Fe sub(5)(MoO sub(4)) sub(24 ) were obtained and its structure was solved (the space group P 1[macr], a=12.5814(5), b=13.8989(5), c=28.4386(9) Aa, alpha =90.108(2), beta =90.064(2), gamma =90.020(2) degree , V=4973.0(3) Aa super(3), Z=2, R=0.0440). Characteristic features of the structure are polyhedral layers composed of pairs of edge-shared FeO sub(6) and (Fe, Na)O sub(6) octahedra, which are connected by bridging М о О sub(4) tetrahedra. The layers share common vertices with bridging М о О sub(4) tetrahedra to form an open 3D framework with the cavities occupied by the Cs super(+) and Na super(+) cations. The compound undergoes first-order phase transformation at 642 K and above this phase transition, electrical conductivity reaches 10 super(-3)-10 super(-2) S cm super(-1). Thus, Na sub(25)Cs sub(8)Fe sub(5)(MoO sub(4)) sub(24 ) may be considered as a promising compound for developing new materials with high ionic conductivity.