New ultra-low thermal expansion materials, based on the NZP structure, have been developed and evaluated for future application in ceramic diesel engines which require exceptional thermal shock resistance. Twenty-four cation substitutions, with electron valence ranging from +1 to +5, were used to replace Na⁺ in the NZP skeletal framework. The linear thermal expansion measurements were conducted for ternary systems up to 1000°C. Quarternary & quinternary compounds were processed using combinations of alkali-alkali, alkali-alkaline earth, and alkaline earth alkaline earth instead of Na⁺. These were evaluated for solid solubility. Linear thermal expansion values were obtained for those compositions having single phase composition. Axial thermal expansion measurements were conducted using high temperature XRD for compounds having low linear thermal expansion value from room temperature to 1400°C. Several ultra-low expansion materials (less than 10x10⁻⁷/°C in absolute value) were found, which also had axial thermal expansion anisotropy. ‘ The effects of ionic size and valence of the substitution elements are discussed to explain the linear thermal expansion behavior and thermal expansion anisotropy investigated in these compounds. The effect of crystallinity is discussed, and the unit-cell equilibrium is suggested to interpret the general thermal expansion behavior occurring in crystalline solids. Secondary material properties for the compounds Rb_0.5Cs_0.5Zr₂(PO₄)₃ & Ca_0.5Mg_0.5Zr₄(PO₄)₆ with linear thermal expansion values of -0.3x10⁻⁷/°C & -5x10⁻⁷/°C, respectively, have also been characterized. / Ph. D.