Extended x-ray absorption fine structure study of the MnAs local structure at the phase transitions

Extended x-ray absorption fine structure measurements at the Mn K absorption edge on MnAs in the temperature range between 250 and 500 K are reported. The temperature dependence of the interatomic Mn-Mn and Mn-As distances is in accordance with the lattice parameter behaviour reported by previous diffraction studies. The Debye-Waller factors of the Mn-Mn and Mn-As first sub-shells are both characterized by an anomaly near the first-order magnetostructural transition at TC. This critical behaviour is superimposed on a temperature dependence well described by a correlated Debye model and is analysed in terms of a cusp-shaped anomaly strictly related to a critical lowering in the phonon frequencies