High-temperature crystal structures of NZP (Na1+xZr2P3−xSixO12) have been determined by x-ray measurements made on single crystals. Thermal expansion of NZP (x = 0.11) parallel to the hexagonal c axis is positive (about 22.4 ⊠ 10−6°C−1 between 25° and 700°C), whereas expansion perpendicular to c is slightly negative (about 5.4 ⊠ 10−6°C−1), resulting in an average volume thermal expansion of 11.8 ⊠ 10−6°C−1. A proposed structural model to interpret this anisotropic thermal expansion of NZP is tested to prove the model's validity. In this model the rotation of the phosphate tetrahedron is coupled to the rotation of the zirconium octahedron. The observed thermal expansions of sodium, zirconium, and phosphorus cation coordination polyhedra are 10.8, 0.00, and −0.23 (all × 10−6°C−1), respectively. The large thermal expansion of the sodium site is offset by rotations in the Zr–P polyhedral framework, thus yielding the low net expansion of NZP.