LiNbO3 (LN)-type polar magnets are technologically important but require stringent and costly high-pressure synthesis with very limited sample yields. We develop a chemical strategy to reduce the physical synthesis pressure. LN-type polar magnets require 7 GPa to stabilize in the high-pressure Mn2FeNbO6 (MFNO) phase. Here, MFNO was successfully stabilized in the isostructural LN matrix at intermediate physical pressure (below 5 GPa) at gram levels for each run by dilution with LN according to (Li1-xMnx)(Fex/2Nb1-x/2)O-3 (x = 0.18, 0.33, 0.46, 0.57). LN-diluted MFNO demonstrates ferromagnetism above room-temperature (magnetic ordering temperature T-C approximate to 516-554 K) and has large estimated spontaneous polarization (P-S approximate to 18-63 mu C/cm(2)). Irreversible c-axis near-zero thermal expansion stemming from magnetostriction was observed around the magnetic transition temperature region upon heating at ambient pressure, which irreversibly elongates the distance between the face-sharing (Li/Mn) and (Fe/Nb) octahedral centroids along the c-axis and thus weakens the magnetic interactions. The magnetic ordering temperature drops in the annealed samples. The findings in (Li1-xMnx)(Fex/2Nb1-x/2)O-3 show that polar magnets can be made by chemical pressure together with soft physical pressure and shed light on large-scale and lower cost stabilization of high-pressure phases.