Anharmonicity and scissoring modes in the negative thermal expansion materials <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ScF</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CaZrF</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:math>
Physical review. B./Physical review. B