Photoluminescence spectra and unusual temperature shift of zero-phonon emission line of V$^{3+}$ in SrTiO$_3$

The paper presents the first observation of photoluminescence spectra of V$^{3+}$ impurity in SrTiO$_3$. The broad band of the observed emission is located in the near IR region and at low temperatures consists of a pronounced zero-phonon line (ZPL) with a maximum at 1157.1 nm (8642 cm$^{-1}$) at 77 K and developed vibronic sidebands extending to 1450 nm. The observed ZPL is associated with an intracenter $^1T_{2g}\to{}^3T_{1g}$ or a closely located $^1E_g\to{}^3T_{1g}$ emission transition in V$^{3+}$ (3d2) ions replacing Ti$^{4+}$ ions. It was found that the temperature shift of zero-phonon line is unusually large and its frequency decreases upon lowering the temperature. The local configuration instability of V$^{3+}$ ions in the $^3T_{1g}$ ground state caused by soft TO1 temperature-dependent phonon mode present in SrTiO$_3$ is considered as a possible source of the observed unusual temperature shift. Keywords: ABO$_3$ incipient ferroelectrics, impurity ions of transition metals, luminescence, zero-phonon lines.