Vibrational Analysis of Paraelectric–Ferroelectric Transition of LiNbO₃: An Ab-Initio Quantum Mechanical Treatment

• Main Authors: Francesco Silvio Gentile, Rosita Diana, Barbara Panunzi, Ugo Caruso, Alexander Platonenko, Fabien Pascale, Roberto Dovesi
• Format: Article
• Language: English
• Published: MDPI AG 2021-09-01
• Series: Symmetry
• Subjects: lithium niobate; ferroelectricity; DFT-simulation; CRYSTAL code; IR spectrum; isotopic substitution
• Online Access: https://www.mdpi.com/2073-8994/13/9/1650

The phase transitions between paraelectric (PE) and ferroelectric (FE) isomorph phases of LiNbO<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>3</mn></msub></semantics></math></inline-formula> have been investigated quantum mechanically by using a Gaussian-type basis set, the B3LYP hybrid functional and the CRYSTAL17 code. The structural, electronic and vibrational properties of the two phases are analyzed. The vibrational frequencies evaluated at the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mo>Γ</mo></semantics></math></inline-formula> point indicate that the paraelectric phase is unstable, with a complex saddle point with four negative eigenvalues. The energy scan of the A<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>2</mn><mi>u</mi></mrow></msub></semantics></math></inline-formula> mode at −215 cm<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></semantics></math></inline-formula> (<i>i</i>215) shows a dumbbell potential with two symmetric minima. The isotopic substitution, performed on the Li and Nb atoms, allows interpretation of the nontrivial mechanism of the phase transition. The ferroelectric phase is more stable than the paraelectric one by 0.32 eV.