125Te NMR chemical shifts and tellurium coordination environments in crystals and glasses in the Ge–As–Sb–Te system

Abstract

The local coordination environments of Te atoms have been investigated in crystalline and glassy binary and ternary tellurides in the system Ge–As–Sb–Te using 125Te solid-state wideline nuclear magnetic resonance (NMR) spectroscopy. The average 125Te NMR chemical shifts in these materials range from 300 to 1050, 90 to 700 and −2000 to −4100ppm for 2, 3 and 6-coordinated environments, respectively. Te atoms are predominantly 2-coordinated in binary Ge–Te, As–Te and ternary Ge–As–Te glasses. The 125Te NMR spectrum of the cubic Ge1Sb2Te4 phase with rock salt structure is consistent with a random distribution of Ge/Sb vacancies in the lattice. Besides the coordination number, the 125Te chemical shifts in these materials are also found to be sensitive to the chemical identity of the nearest neighbors. 125Te NMR spectroscopy shows significant future promise in its application as a technique complementary to diffraction and EXAFS in understanding the short-range structure of amorphous Ge–As–Sb tellurides.