Long Electron Spin Coherence Times of Atomic Hydrogen Trapped in Silsesquioxane Cages

Encapsulated atomic hydrogen in cube-shaped octa-silsesquioxane (POSS) cages of the Si$_8$O$_{12}$R$_8$ type (where R is an organic group) is the simplest alternative stable system to paramagnetic endohedral fullerenes (N@C$_{60}$ or P@C$_{60}$) that have been regarded as key elements of spin-based quantum technologies. Apart from common sources of decoherence like nuclear spin and spectral diffusion, all H@POSS species studied so far suffer from additional shortening of $T_2$ at low temperatures due to methyl group rotations. Here we eliminate this factor for the first time by studying the relaxation properties of the smallest methyl-free derivative of this family with R=H, namely H@T$_8$H$_8$. We suppress nuclear spin diffusion by applying dynamical decoupling methods and we measure electron spin coherence times $T_2$ up to 280 $\pm$ 76 $\mu$s at $T=90$ K. We observe a linear dependence of the decoherence rate $1/T_2$ on trapped hydrogen concentrations ranging between 9$\times 10^{14}$ cm$^{-3}$ and 5$\times 10^{15}$ cm$^{-3}$ which we attribute to the spin dephasing mechanism of instantaneous diffusion and a nonuniform spatial distribution of encapsulated H atoms.