Thermodynamic characteristics of Li2MoO4, Li2W0.85Mo0.15O4 single crystals and stability direction for alkali molybdates

Single crystals of Li2MoO4 and Li2W0.85Mo0.15O4, as perspective materials to search for neutrino-less double beta decay, have been grown by low-temperature-gradient Czochralski technique (LTG CZ). The standard formation enthalpies for single crystals of Li2MoO4 and Li2W0.85Mo0.15O4 have been determined by two methods: (i) solution calorimetry in KOH ( Δ f H 0 (Li2MoO4, s, 298.15 K) = −(1521.9 ± 1.9) kJ mol−1) and (ii) high-temperature reaction calorimetry in sodium molybdate melt (3Na2O · 4MoO3) ( Δ f H 0 (Li2MoO4, s, 298.15 K) = −(1529.1 ± 7.3) kJ mol−1; Δ f H 0 (Li2W0.85Mo0.15O4, s, 298.15 K) = −(1597.8 ± 4.3) kJ mol−1). The measured solution enthalpies of Li2CO3, MoO3, WO3, Li2MoO4, Li2W0.85Mo0.15O4 in 3Na2O · 4MoO3 melt and solution enthalpies of Li2CO3, MoO3, Li2MoO4 in KOH have been used to calculate Δ f H 0 . The thermodynamic stability of A2MoO4 (A = Li, Na, K, Rb, Cs) with respect to simple oxides mixture has been determined. It has been found that the relative thermodynamic stability is increased from Li2MoO4 up to Cs2MoO4. This fact seems to be important from practical point of view. The lattice enthalpies for A2MoO4 compounds have been calculated using the Born-Haber cycle. The dependences of lattice enthalpies for A2MoO4 on structural parameters have been constructed, which allow one to predict the thermodynamics for compounds not investigated.