an internal pressure of 46 atm of oxygen was established after 3 hours the temperature inside the autoclave was 340 u00b0 c and that temperature was maintained for 120 minutes the final pressure amounted to 250 atm the magnetic characteristics of the obtained product were as follows hci 480 oersteds u03c3 s 79 1 e m u g u03c3 r u03c3 s 0 55 the procedure followed in this example was the same as in example 1 except that the amount of sb 2 o 3 introduced into the chromium chromate solution was 0 239 g and thus the percent by weight of antimony amounted to 0 4 with respect to the anhydrous chromium chromate in the hollow space between the test tube and the autoclave walls no distilled water was introduced the autoclave was heated in a muffle oven thermostabilized at a temperature of 380 u00b0 c at the beginning of the example an internal pressure of 85 atm was established by means of nitrogen added from a nitrogen bottle after 3 hours the temperature inside the autoclave reached 375 u00b0 c and was maintained thereat for 120 minutes the final pressure amounted to 350 atm the magnetic characteristics of the obtained product were hci 490 oersted u03c3 s 87 3 e m u g u03c3 r u03c3 s 0 54 the following examples are given to show the preparation of cro 2 modified by the inclusion of a second element in addition to sb in examples 17 24 this second element is la in examples 25 28 this second element is respectively li sr fe and pb the procedures followed in these examples were the same as in example 1 except that before introducing the sb 2 o 3 into the chromium chromate.