Thermodynamic and Structural Properties of Sodium Lithium Niobate Solid Solutions

Thermodynamics of the Na 1−x Li x NbO 3 system is investigated by high‐temperature drop‐solution calorimetry in molten 3Na 2 O–4MoO 3 solvent at 973 K. Standard molar enthalpies of formation are derived. The estimated heats of transition between hypothetical and stable structures, lithium niobate and perovskite for NaNbO 3 and vice versa for LiNbO 3 are −6 kJ/mol and −10 kJ/mol, respectively. X‐ray diffraction studies at room temperature showed for 0 ≤ x ≤ 0.14 there are three phases based on different ordering of the perovskite type lattice: orthorhombic with a quadrupled reduced perovskite cell at 0 ≤ x ≤ 0.02, orthorhombic with a doubled reduced perovskite cell at 0.015 ≤ x ≤ 0.14, and rhombohedral at 0.08 ≤ x ≤ 0.13. There are two two‐phase (morphotropic) regions with coexistence of the two orthorhombic phases at 0.015 ≤ x ≤ 0.02 and with the second orthorhombic phase coexisting with the rhombohedral phase at 0.08 ≤ x ≤ 0.13. A reproducible anomaly in specific heat at ∼600 K, not reported previously, has been observed in pure NaNbO 3 . Heat‐capacity measurements confirm a phase transition at 553 K for 0.07 ≤ x ≤ 0.09. With increasing lithium concentration, a gradual disappearance of high‐temperature phase transitions associated with tilting of oxygen octahedra has been observed.