Structural investigations of ferecrystals [(SnSe)&lt;inf&gt;1+&amp;#x03B4;&lt;/inf&gt;]&lt;inf&gt;m&lt;/inf&gt;[TSe&lt;inf&gt;2&lt;/inf&gt;]&lt;inf&gt;n&lt;/inf&gt; (T = Mo, Ta) by means of transmission electron microscopy