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Using mainly numerical methods, we investigate the ground-state phase diagramof the S=2 quantum spin chain described by $H = \sum_j (S_j^x S_{j+1}^x + S_j^yS_{j+1}^y + \Delta S_j^z S_{j+1}^z) + D \sum_j (S_j^z)^2$, where $\Delta$denotes the $XXZ$ anisotropy parameter of the nearest-neighbor interactions and$D$ the on-site anisotropy parameter. We restrict ourselves to the case with$\Delta \ge 0$ and $D \ge 0$ for simplicity. Each of the phase boundary linesis determined by the level spectroscopy or the phenomenological renormalizationanalysis of numerical results of exact-diagonalization calculations. Theresulting phase diagram on the $\Delta$-$D$ plane consists of four phases; theXY 1 phase, the Haldane/large-$D$ phase, the intermediate-$D$ phase and theN\'eel phase. The remarkable natures of the phase diagram are: (1) the Haldanestate and the large-$D$ state belong to the same phase; (2) there exists theintermediate-$D$ phase which was predicted by Oshikawa in 1992; (3) the shapeof the phase diagram on the $\Delta$-$D$ plane is different from that believedso far. We note that this is the first report of the observation of theintermediate-$D$ phase.

Haldane, Large-D and Intermediate-D States in an S=2 Quantum Spin Chain with On-Site and XXZ Anisotropies