The Lanthanum and Actinium trouble

I would be grateful if you answer this question for me friends:

According to the Aufbau Principle after the 6s orbital is filled 4f fills up and only then 5d gets filled. But this doesn't happen so. In barium the 6s orbital is complete and this according to the Aufbau Principle means that electron has to enter 4f. but this is not so.the element next to Barium is Lanthanum where the last electron enters 5d and not 4f.This is a violation of Aufbau Principle I have learnt that the same happens in Actinium also. WHY DOES THIS VIOLATION OCCUR?...Please be kind to state your exact reasons too...

#Chemistry

Easy Math Editor

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Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$