Studies of 27Al NMR in SrAl4

A charge density wave (CDW) transition at T-CDW = 243 K and a structural phase (SP) transition at approximately 100 K occur in SrAl4 with the BaAl4-type body center tetragonal structure, which is the divalent and non-4f electron reference compound of EuAl4. To understand the behaviors of the CDW and SP transitions, the Al-27 NMR measurements using a single crystal and a powder sample of SrAl4 have been carried out. The line width below T-CDW is modulated by an electrical quadruple interaction between Al-27 nucleus and CDW charge modulation. The incommensurate CDW state below T-CDW changes into a different structure below T-SP. The temperature dependences of Knight shifts of Al-27(I) and Al-27(II) show the different behaviors. The temperature variation of Al-27(I) Knight shift shows anomalies at the CDW and SP transition temperatures, revealing the shift to negative side below T-CDW, which is attributable to the core polarization of the d-electrons. However, Al-27(II) Knight shift keeps almost constant except for the small shift due to the SP transition. The 1/T1T of Al-27(I) indicates the obvious changes due to the CDW and SP transitions, while that of Al-27(II) takes a constant value. The density of state at the Fermi level at Al(I) site below 60 K would be about 0.9 times less than that above T-CDW.