Pengaruh substitusi Mg di situs Fe terhadap struktur kristal, sifat listrik dan optik material perovskite LaFeO3 (LaFe1-xMgxFeO3 dengan x = 0.1, 0.2, dan 0.3) = The influence of Mg-substitution in Fe-site to the crystal structure, electrical properties and optical properties of material perovskite LaFeO3 (LaFe1-xMgxFeO3 with x = 0.1, 0.2, 0.3)