Paramagnetic susceptibility of semiconducting CdF2:In crystals: Direct evidence of the negative-U nature of the DX-like center

Paramagnetic susceptibility ${\kappa }_{\mathrm{para}}$ of ${\mathrm{CdF}}_{\mathrm{2}}$ crystals with bistable In centers is measured in the temperature range $T$ =4–300 K. For crystals cooled in the dark down to liquid helium temperature, ${\kappa }_{\mathrm{para}}$ is determined by the trace ${\mathrm{Mn}}^{\mathrm{2+}}$ impurity. Illumination of the sample by the ultraviolet-visible light results in the appearance of the photoinduced ${\mathrm{\delta \kappa }}_{\mathrm{para}}$ signal due to formation of centers with the magnetic moment $\text{J=1/2}$ . This gives a clear evidence of an absence of paramagnetism in the deep state of the bistable In center and its presence in the shallow state, i.e., proves the negative-$U$ nature of the deep state.