High Pressure study of Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O with x = 0, 0.10, and 0.15

Here we report a systematic study of the effects of pressure on the resistivity for the superconducting and spin/charge density wave (SDW/CDW) transitions of Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O (x = 0, 0.10, and 0.15). With increasing pressure no measurable change is observed for the SDW/CDW transition temperature (T$_S$) for x = 0.15; however, for x = 0 and 0.10 a decrease of the SDW/CDW transition temperature T$_S$ is observed. With respect to the superconducting transition temperature T$_c$, the effects of pressure effect on the three samples are different. The T$_c$ of BaTi$_2$Sb$_2$O increases linearly from 1.2 K to ~ 2.9 K at 16.1 kbars. In contrast, T$_c$ of Ba$_{0.90}$Na$_{0.10}$Ti$_2$Sb$_2$O only initially increases to 4.2 K and then saturates at higher pressure values. For Ba$_{0.85}$Na$_{0.15}$Ti$_2$Sb$_2$O, T$_c$ continuously decreases with increasing pressure.