Antimony (element), stibium
銻 centers on the metallic element antimony, also known historically as stibium. Both given meanings refer to the same chemical substance.
銻 combines 金 (metal) with 弟, which likely contributes the sound. The character was created to name the metallic element antimony.
The metal radical 金 on the left marks this as a metallic element. The right side 弟 (younger brother) can be imagined as a younger, less common metal: antimony.
For テイ, imagine a tea (ティー) made from antimony—a strange metallic brew. Tea -> テイ, and the metal 金 reminds you it's a metallic element.