Low‐Dimensional Zero Thermal Expansion Enables Ultrastable Thermo‐Optics in YAl ₃ (BO ₃ ) ₄ Single Crystal

ABSTRACT Zero‐thermal‐expansion (ZTE) materials are critical for stabilizing device performance under thermal fluctuations. Traditionally, achieving enhanced thermal stability has required three‐dimensional (3D) ZTE, a stringent condition that severely limits material availability. Here, we show that low‐dimensional ZTE can be realized in single crystals by orienting ubiquitous bond rotations along specific crystallographic axes in the framework structures, thus relaxing the strict requirements for 3D ZTE. This approach enables a two‐dimensional ZTE response, as demonstrated in a trigonal YAl 3 (BO 3 ) 4 (YAB) crystal, which exhibits an ultralow thermal expansion coefficient of −0.02(7) MK −1 within the ab plane from 83 to 180 K. The practical impact of this low‐dimensional ZTE is evidenced by the thermo‐optical performance of YAB crystal: within the ZTE temperature range, the thermo‐optical coefficient varies by only 0.03(2) × 10 −8 K −2 in the ZTE plane, approximately two orders of magnitude lower than those of conventional optical materials. This study establishes a new design strategy for functional ZTE materials and highlights the application potential of dimensionally confined ZTE effects in precision optical and electronic devices.