Thermodynamic and Structural Properties of Sodium Lithium Niobate Solid Solutions

• Published in: Journal of the American Ceramic Society 2002-02, Vol.85 (2), p.379-384
• Main Authors: Pozdnyakova, Irina, Navrotsky, Alexandra, Shilkina, Lidia, Reznitchenko, Larisa
• Format: Article
• Language: English
• Subjects: lithium niobate; sodium/sodium compounds; structure; thermodynamics
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/j.1151-2916.2002.tb00100.x

Thermodynamics of the Na1−xLixNbO3 system is investigated by high‐temperature drop‐solution calorimetry in molten 3Na2O–4MoO3 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 NaNbO3 and vice versa for LiNbO3 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 NaNbO3. 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.