The double perovskite ${rm La}_2{rm NiTiO}_6$ is identified as a three-dimensional $S=1$ quantum magnet. By means of Density Functional Theory we demonstrate that this material is a high-spin $d$-electron system deep in the Heisenberg limit and establish that its paramagnetic Mott phase persists down to low temperatures ($T_{rm N}$=25K) not because of frustration effects but rather for the extreme strong coupling physics. Our many-body calculations on an $ab$ $initio$-derived multi-orbital basis predict indeed a kinetic energy gain when entering the magnetically ordered phase. ${rm La}_2{rm NiTiO}_6$ emerges thus as a paradigmatic realization of a spin-triplet Mott insulator. Its peculiar properties may turn out to be instrumental in the ongoing chase after correlated topological states of matter.
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