The Lanthanide contraction which is the decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, is due to poor shielding of the 4f electrons which decreases the radius. It is also known that the relativistic effects make the core orbitals (s,p) contract while the f and d orbitals expand. How does this "relativistic effect" leads to the contraction of these orbitals which lead to Lanthanide contraction?

Dear Sourangshu Ghosh thank you for this very interesting question. Yes, the lanthanide contraction is at least partially a relativistic effect. This has been studied in great detail by my Finnish colleague Pekka Pyykkö who described this in an excellent review article entitled

Relativistic Effects in Chemistry: More Common Than You Thought

Article Relativistic Effects in Chemistry: More Common Than You Thought

Fortunately this very instructive article is freely available as public full text on RG. It has an extra chapter on lanthanides and actinides.

Yet another (somewhat older) article was published by leading theoretical chemists which is entitled

Lanthanide and Actinide Contractions: Relativistic and Shell Structure Effects

Article Lanthanide and Actinide Contractions: Relativistic and Shell...

This paper can bee freely downloaded from the public internet. For your information lease find attached a pdf file of this article.

Dear Sourangshu Ghosh thank you for this very interesting question. Yes, the lanthanide contraction is at least partially a relativistic effect. This has been studied in great detail by my Finnish colleague Pekka Pyykkö who described this in an excellent review article entitled

Relativistic Effects in Chemistry: More Common Than You Thought

Article Relativistic Effects in Chemistry: More Common Than You Thought

Fortunately this very instructive article is freely available as public full text on RG. It has an extra chapter on lanthanides and actinides.

Yet another (somewhat older) article was published by leading theoretical chemists which is entitled

Lanthanide and Actinide Contractions: Relativistic and Shell Structure Effects

Article Lanthanide and Actinide Contractions: Relativistic and Shell...

This paper can bee freely downloaded from the public internet. For your information lease find attached a pdf file of this article.

Frank T. Edelmann thank you for the excellent answer, sir. I downloaded these 2 papers, sir. It is very interesting to read. I appreciate your kindness to share these articles, sir.