A series of LiMnyFe1-yPO4 samples have been prepared in the whole range 0≤y≤1. Chemical delithiation could be achieved to obtain MnyFe1-yPO4 in the range 0≤y≤0.8, keeping the same crystal phase (olivine structure, space group Pnma). The composition y=0.8 is the limit where the delithiated phase is still crystallized, but abruptly suffers strains at the molecular scale evidenced by both optical spectroscopy and X-ray diffraction. The analysis of the magnetic properties shows that all the samples are the concentration of impurities is negligible. The concentration of polarons, either holes associated to Li vacancies in LiMnyFe1-yPO4 or electrons associated to the existence of Li left in the matrix of MnyFe1-yPO4, is found to be small (≤ 1%) in all the samples. For y ≤0.6, all the Mn3+ ions MnyFe1-yPO4 are in the high spin state (S=2). At larger manganese concentration, however, the Mn3+ ions in excess of the critical concentration yc=0.6 undergo a transition to the low-spin state (S=1). As a consequence, and in contrast with prior works, we find that Mn0.8Fe0.2PO4 has magnetic interactions that are much smaller, and no antiferromagnetic ordering in this compound is detected, at least above 20 K. Antiferromagnetic ordering that had been reported so far for MnyFe1-yPO4 at large y-composition might come from incomplete delithiation. The spin transition of Mn3+ in concentration (y-yc) to the low-spin state is at the origin of the strain fields at the molecular scale that increase with y for y>0.6, and ultimately prevents the full delithiation for y>0.8. This result sheds light on the reason for the degradation of cathode properties in Mn-rich compounds of the heterosite-purpurite series, while the electrochemical properties are good in the range y ≤0.6 but only at slow rates, due to the very small hopping mobility of the small polaron.